For the following procedure first back up your Windows Registry, just in case something goes wrong.
Here is a nice program that will do that and restore the registry also if needed.
http://www.larshederer.homepage.t-online.de/erunt/index.htm

Download Windows Installer CleanUp Utility from here:
http://support.microsoft.com/default.aspx?scid=kb;en-us;290301

If you find the game you want to reinstall listed, click on it and remove it. Be carefull not to remove anything else.You should then be able to reinstall the game.
If not click start>run> and type regedit
click edit>find and type in the name of the game
then delete the result, make sure it's the targeted game.
Press F3 to search more for other entries and delete.

Good Luck.

You could try using System Restore (Programs / Accessories / System Tools / System Restore) and restore to a date before this happened - this could solve your problem. If not, you could try the Windows Installer Cleanup Utility available here.

I've used it before without problem but I'd advise creating a Restore Point before you use anything that modifies the registry - I use the free application ERUNT (see my signature) to back up my registry every day and this is an even better safety net in case of registry problems.

Thanks for the replies, I deleted the game files from the reg and now it's giving me the option to install, thanks guys!

Good for you - you now have some real insurance against future problems. Good luck.

Don't worry I followed what you said and installed and ran that before going on my deleting spree, thnaks again.

And you will install ERUNT, won't you? ;>)