I'm hearing system clicks (like a window closing, the default windows 'click') at random intervals and occasionally hearing sounds typically associated with pop-ups, like a recorded voice saying "You've won a new laptop!" etc etc when I'm not using the web at all.

Avira and Spyware Doctor put up warnings of blocked cookies a few times a day as well.

I use Chrome as my default browser, if that makes a difference. The last time I actually loaded IE for anything browsing related was nearly a year ago.

Logs are attached

checking your logs

YOUR CLEAN good job but i did find a security issue some Flash BHO originating in 2004 2005

you can use the ANTI.log i am posting and run Hijackthis one more time. check this and fix them

if you need flash update FLASH HERE

happy sorfing and never click on a bad window ALT/F4 it
Wolf

Just now, before I checked the forums, I heard another recording telling me I'd won a free Nintendo. My browser was closed and the only app I was actively using was Photoshop CS3.

Glad to hear things look good but something must still be buried in my system...any ideas?

did you remove the flas items i suggested
also did you have a web site open ?

I've removed the items you suggested but I haven't been at my PC since. At the time the only application I had running was Photoshop so no websites were open.

I'll post again tomorrow morning and let you know if anything pops up.

Thanks for your help! Hope you're not too snowed in, I'm sitting under almost 7 inches of fresh powder from this afternoon.

I'm quite sure something is wrong now. Upon waking up this morning I had a blocked cookie notice from Spyware Doctor (my pc was left on overnight). It was trying to save it to:

C:\Documents and Settings\NetworkService.NT AUTHORITY\Local Settings\Temporary Internet Files

I went and checked that folder and sure enough it's FULL of things from java scripts, images, websites...all things from websites I've never been to.

That seems clean. Nothing wrong with that.

You can delete those and not to worry about blocked cookies it is a good thing

HOWEVER
C:\Documents and Settings\NetworkService.NT AUTHORITY\Local Settings\Temporary Internet Files

NT AUTHORITY is usually a domain / domain user account that is not on a single user WinXP machine.

Have you used or do user any remote assistence, member of a domain ???