Ad-aware usually kills most of that shit.
It might be a spyware that was downloaded to your computer and is starting up without you knowing though. The 'easy' way to get rid of those kinda programs, download a copy of TWEAK-XP, then remove all the shit from your 'autostarts' ...
There is another way to do it without downloading Tweakxp, but its a bit complicated.
The last thing you can do is double check your computer for virus's as well, goto
www.mcafee.com and use the free virus scanner on the right hand side. It wont fix the virus's, but it will tell you if you have any or not.