I use a combination of
Spywareblaser and
spybot to keep them away.
To find them you can use
Adaware to find & get rid of them.
You may find that you have this file C:\Program Files\Preview AdService\PrevAdKeep.exe etc on your HDD.
have a search on Google for the file you mentioned.
Try
HijackThis available from
here to identify what is running on your system.
Hijack this, spybot & adaware should help get rid, spybot/spywareblaster should help keep em at bay.
You might also want to consider a firewall such as
Zonealarm and an antivirus package such as
AVG.
At the very least, you should be running these 2 types of program.
All these programs are freeware & work very well.