kevsta, usually when I hear about this type of gremlin, the first thing that pops into my mind is virus/trojan/spyware.
Run online scans here (most require IE with activeX):
a-squared Web Malware Scanner - Scan and clean your computer from Trojans, Worms, Dialers, Keyloggers and Spyware/Adware for free!
Free Virus Scan - Kaspersky Lab
The reason you do this online even if you have an AV installed is because one of the first things some of the nasty trojans do is shut off your AV, but then make it behave as if it's still running. The online scan will let you know if that's happened.
I run these online scans a couple times a month as a backup check.