I agree with Mac70 about anyispyshield
Personally - I'd cut my losses
by all means try spybot search and destroy
http://www.safer-networking.org/en/download/in dex.html
AND ad-aware
http://www.lavasoftusa.com/products/ad_aware_f ree.php
between them they will generally do a good job
(use them at least once a week)
If there is no joy there ... the secutity websites will almost certainly get there in the end ...
but it's like linux users ... they tend to loose sight of the time ... and simply rise to the challenge.
if you do have something better to do ... copy anything important to CD or DVD ... and seriously consider rebuilding.
if you've used the machine for more than a year .... and regularly visit the type of sites downloaders come from ... a fresh start is probably in order.
after all you may have something you don't know about working in the background (from what you say) - a downloader should hardly be noticeable ... after all ... that's their job!
but can take much to-ing and fro-ing before you get there
but