Virus on my computer? Help needed

[Norway]

I've had some nasty computer problems recently:

When I browse Wikipedia, some pictures in an article are replaced by a typical Windows XP warning sign (kind of a red circle with a white X in it, with some random error info beside it). If I click ths it leads me to either some porn site or a Norwegian advertisement (probably meaning my IP address is being hacked?).

I can't use Google. Other search engines can be used, however they run very, very slow.

When I use Firefox, often a porn site appears in a new tab. This often happend when I use Wikipedia.

I downloaded a game demo a while ago, but when AVG Free said that the demo had a trojan horse in it, I put it in Recycle Bin, entered Recycle Bin, highlighted the file and pressed delete, then I confirmed the delete... Is this enough?

Thanks for all your help!

EDIT: Also, when I visit sites that have advertisements on the side, the advertisements often appears as messages telling me that I have a virus or that I might be infected with spyware, they are clickable but of course I don't click on them.

EDIT 2: When I try deleting some of the suspicious files from the computer, Windows Vista tells me I can't although I'm Administrator of the computer.

EDIT 3: I'm scanning through the PC with AVG Free, after about 17 minutes it has found this:

File: HKLM/SOFTWARE/Omniquad

Result/Infection: Found Adware.RogueSuspect

[uninstall.exe]

Check this link out and back your information up!

adware.roguesuspect FP? - Wilders Security Forums

[aRIfA4]

You're gonna need spybot or/and ad-aware also.

install them both and update them then boot in safe mode and unplug the ethernet cable /disable wireless. then do full scan.

[imanwnms]

Here I may offer a help.
Do all of the downloads and writing to disk actions at a friends computer.
Uninstall of of the browsers except Internet Explorer.
Uninstall software you dont use.
Then Restart your computer.
After that delete all your browsing history by clicking Ctrl+Shift+Del together. Then click delete all and delete all history.
Then do a Disk Cleanup from Start>Programs>Accessories>System tools>Disk cleanup.
After that empty files in your recycle bin.
Check for updates to your softwares by downloading the software update checker at Update Checker - AppHit.com.
Update your computer by going to control panel> Windows Update.
If you use XP SP2 update to 3 at Download details: Windows XP Service Pack 3 - ISO-9660 CD Image File by writing the files to a disk.
After updating your computer none of your files will be damaged.(Still do a backup).
At last

[imanwnms]

Here I may offer a help.
Do all of the downloads and writing to disk actions at a friends computer.
If you use XP SP2 update to 3 at Download details: Windows XP Service Pack 3 - ISO-9660 CD Image File by writing the files to a disk.
After updating your computer none of your files will be damaged.(Still do a backup).
Uninstall of of the browsers except Internet Explorer.
Uninstall software you dont use.
Then Restart your computer.
After that delete all your browsing history by clicking Ctrl+Shift+Del together. Then click delete all and delete all history.
Then do a Disk Cleanup from Start>Programs>Accessories>System tools>Disk cleanup.
After that empty files in your recycle bin.
Check for updates to your softwares by downloading the software update checker at Update Checker - AppHit.com.
Update your computer by going to control panel> Windows Update.
At last do a disk defragment.
Computer will be super fast.
Reply me at imanwnms@gmail.com