C Drive Memory getting eaten up

Hey there,

I realized a few days ago while I was trying to defrag that I had less than 1% of memory left on my C Drive. Funny thing is I havent installed anything on it in a long time and last time I checked I had at least 10 gb free. I also realised that I had apparently downloaded 8 gb last moth from the internet which I am pretty sure is impossible. I think my pc, Windows XP, may be infected with some malware which just keeps downloading random files and installs them onto my computer, could this be possible? What is there I can do?

Ive run several virus checks and spyware checks but found nothing that helps, after deleting several gigs of files I find that the memory keeps filling again.

As well, if this might help, I find that my pc is installing "windows updates" much more often than usual. Is a virus possibly masking its downloads as windows updates?

Thanks in advance