mysterious increase in disk space usage

Have recently experienced a mysterious increase in amount of
disk space used when checking properties of C drive in My Computer.
My disk space increased about 700 MB in a couple of days even
though haven't added any files or programs. Am using XP Pro w\ all
updates installed. Ran the disk cleanup utility to no avail. Also
did chkdsk \ degrag etc & no change. Did a google search on this &
could only find suggestions for several 3rd party utilities to
download to monitor disk space usage. Are there any of these that
are recommended by the gurus here?
Any thoughts on what would cause this. Has anyone heard of a
virus \trojan or spyware that would cause this. The machine is
running just fine otherwise. Spybot \ NAV haven't shown a thing.
Thanks,
dc
.