But ok, you clearly are Hijacked. but I have few questions.
When you go to myspace, does it show up normal? I mean like the front page of myspace changes everyday, does the page you see as “myspace” change every day? or does it always stay the same? If it stays the same then you are going through a fake dns, so he ( the hacker) has a fake myspace. If it changes each day then you just have an adsense gamer. Means he uses google’s advertisement company and figured out how to cheat the system and turn everything you do on your computer, into $$ for him.
If it’s the ladder then you can cross your fingers and hope I am right in saying that he is not going for your personal info he just wants you to see the advertisements he gets paid for displaying. but don’t bet your soul on it. Some people are sick . Sorry to inform you of that. That’s just the most likely. So still do not use your credit card on that machine, and do not use your social security number on it either.
If it is the first one I mentioned, then he sees everything you do and everything you type. Passwords credit card numbers and so on. Everything.
but here is what I am going to do to try and help you on this issue.

First things first http://www.mvps.org/winhelp2002/hosts.htm go to that page. No it is not the be all end all, but boy it sure blocks your computer from connecting to many many bad websites. Scroll down the page until you see the folder that looks like files being dropped into it. click on the “Download: hosts.zip ” download that take all files out of that zip folder and save them to the desktop, they have to be outside of the zip folder for them to function properly. double click on the “mvps” file. two seconds will pass and the pop up window will turn blue, hit your space bar, you are done. Close all web browsers so the hosts file can reset it’s self and get a fresh new read of banned websites.
now that the bad sites are blocked (so you do not get further infected). Let’s go get that thing scanned for a virus or malware of any sort. Bitdefender
http://www.bitdefender.com/scanner/online/free.html twice during the process of the scan, you have to click that stupid bar that explorer has so you can authorize the activeX file (web browser software to act like an actual program)to run. run the full scan. If it does not run, then you do not have administrator rights.

Either way, next move is
a-squared Free 4.5 or greater from http://www.emsisoft.com/en/software/download/
Free just go to that page and scroll down, download that program and scan with it. full scan, go to the store, it will take a few minutes. after the first scan, click on “update” you will get new update list , scan again.

There re a few programs on the asquared site that have trial versions, can’t hurt to try them, but wait till we go through the hitlist of tools that are a must first.

http://www.malwarebytes.org/ click on “download free version” get it run it.

bow that http://www.superantispyware.com/ I hate that name, but man it actually worked. If found what was called a “rootkit” when you get rooted, well… you are screwed most virus scanners will not find it. super ant spyware actually found it. If it will not let you download as if the link is dead, then the dns trick he is pulling blocks that download, message me I will download it and put it on a free server site that he himself is probably using (so he can’t block that place, or he will block himself) to make it better available for you. if you are infected, it can’t get much worse unless you use/used your credit card or social security or gave home address information etc etc etc …..

good chance you may have Vundo, http://majorgeeks.com/VundoFix_d4954.html would not kill you to scan with that.

If you have Norton or any other virus scanner I am sorry to inform you that they lead you into a false sense of security. Most people who write malware, write it to specifically attach it’s self to the virus scanner. Why? because the virus scanner will not remove it’s self or delete any of it’s own files, this leaves the virus in control of the virus scanner. The virus commands the door open for all sorts of junk the virus scanner or blocker would be blocking, and it just goes crazy without you seeing much of any pop ups. It’s all automatic and happens 900000 times a day to millions of computers.

Your best protection, is safe surfing. If you see a pop up saying that you have a virus and need to scan, but the pop up looks nothing like your virus scanner, or you have no virus scanner yet get a notification that you have a virus, well DON’T CLICK THE NOTIFICATION. Go to your taskbar and right click every little window button and tell it to “close” do not save anything. It’s better to lose your 7 hours of hard work than to lose your identity.
You get an Email that says you have won something that you did not enter, delete delete delete. or an email about some dude in a far away country with a lot more money than you can imagine wants to wire it to your bank account and he will pinch you off a bit of money once you wire it to his new account. yeah delete that email that is trash. that scam works every single day 100 times a day. You give him your bank info, and then you hear nothing from him. You use your credit card the next day and it is refused, wiped out and in debt..

Truthfully, either way, your computer is compromised. There is no way to ever tell how much is installed on your computer and if you ever got it all.

i am running windows vista, and windows explorer 8…i did the same as you, when the window popped up saying mircrosoft is ready to install windows explorer 8 i clicked on it, but i already was using windows explorer 7, not firefox. i have done a virus scan, (using mcafee) and it showed that nothying was wrong…

please help :)