Have two laptops, off internet for a week, both Dell's XP-SP3 patched current, running SAV 9. Both have similar symptoms. Before boot error, get warning of low drive space (> 1GB available on one, >60GB on other) Will boot Safe Mode, Tried Chkdsk, no Joy. Did a ghost to preserve before flatten, got message about incorrect file size report on temp internet - remove the ie5. folder restarted ok, until next boot. Safe mode V_scan doesn't find anything and V-def's are 11-06 and 11-07 on the two beasties.
I had this issue on 6 computers at 2 different buildings. I was able to fix the issue by restarting into Safe Mode, then running a chkdsk /fix. After reboot I could log in normally. Then uninstall SAV 9 or 10. That is the culprit.
When I initially saw this issue, I was able to fix it with just the chkdsk /fix, but the issue came back within a day or two. I haven't had the issue over the past week, since removing SAV, and installing an updated Symantec Endpoint Protection.
I saw this page, and dashman's response helped me narrow down the problem, since that was one of the common apps.