A few questions
Has your Homepage changed over David?
Have you got any of the following Programs installed?
Spywareblaster(
www.javacoolsoftware.com )
CWShredder
www.intermute.com/spysubtract/cwshredder_download.html Microsoft AntiSpyware (new Title) was GiantSoftware Companies program but has been re released as a free BETA download last week(Very strong) Its a cross between AdAware and Spywareblaster
Both Spywareblaster and the new Microsoft AntiSpyware programs have preset Homepage settings to return the browser to the original setting if need be(works in the backgound)
The CWshredder program "blanks" the Page if there is a detection !

So you have to go to the Internet Explorer Option page to reset the page manually.
The Spywareblaster program is a defence program not a removal program OK?
If after running these programs you
still have Problems I recommend that you download and run the scan on this program
HijackThis then copy the findings (full instructions in text) Next go to this forum Read top posting (locked) for instructions(have to register to submit)Post your logfile should get a reply within a few hours (busy forum) of what to untick on the list.
go to this UK forumDon't be tempted to do it yourself because you tick something it's gone, wrong thing... Then you may have to reload!

this is an experts job)
Only use this if none of the other ideas work OK? Hope it does not get this far?