Possible virus

I recently purchased the game Halo for my computer. After installing the game I tried to play it. After pressing the play button a license agreement is supposed to pop up then the game demos start. However, when I press the play button it will not bring up the license agreement and the game won't launch. I have tried fixing this by uninstalling the game then reinstalling it. I downloaded and ran many different anti-virus and spyware programs per the instructions found on this site. I also downloaded a patch that is supposed to fix this problem – with no luck. My dad and other friends have the same computer as me and the game runs fine on theirs. I also ran a test to see if my computer (Win XP) was able to run the game. It passed the test as I expected it would. Regardless of what I try, the game will not launch. I suspect it may be virus related though.If anyone has any ideas or suggestions any and all help will be greatly appreciated.
Thanks.

 

Thanks for the reply! I initially tried posting this on the games forum but out of eighty people no one had an idea of what was wrong. So I tried posting this here to see if anyone had encountered this problem or something like this before.
Anyways, I tried downloading Ad-Aware before I found the anti-spyware solutions here.
The Ad-Aware would not open. It just sat there like nothing was happening. But after following the procedures for the malware I double clicked the Ad-aware executable and it worked.

 

Oop's, sorry - I obviously forgot an important detail. I got the Ad-Aware working fine but Halo still wouldn't. I keep wanting to reformat my hard drive and start from scratch but my dad says because everything else works fine it seems a bit extreme just for a GAME. My dad just doesn't understand!!! Thanks for keeping with me on this and sorry for not being clear on the last reply.

 

I ran through the procedure and at the end found the virus that was causing the problem. Thanks so much for your help!