Disk free space disapearing

Recently I started to notice my free space on my hd decrease when the machine is idle. Overnight it would loose 5-10GB+ freespace. I have run virus and trojan checks, no logs are running.No unrecogized processes or programs running. No new startups noted.
Also recently (yesterday) lost all stored settings to several programs along with my restore points. No one was on this computer during that time.
Any suggestions??
Running:
WinXP SP2 seagate 40GB, 512 RAM, linksys router on 2 computer, 2 printers and scanner on network with dsl.