If I had external devices of any sort...flash drive, printer, whatever...I would disconnect same from system (after shutting down).
Then I would attempt to run chkdsk /r from the Recovery Console...and then boot into XP and run sfc /scannow. If it's an XP file error, then chkdsk /r may be able to handle it...if it's a missing or damaged system files problem, then sfc /scannow ought to take care of it.
If it's a malware problem (always a possibility), then the system needs to be properly scanned to try to eliminate that as a possible cause.
Please try a couple of these free online scanners to see if anything has slipped by your protection:
(Be advised that some of these scanners will pickup things in "quarantine" from other anti-virus programs - so review the results carefully)http://www.pandasecurity.com/homeusers/solutions/activescan/http://us.mcafee.com/root/mfs/default.asphttp://housecall.trendmicro.comhttp://www.bitdefender.com/scan8/ie.htmlhttp://support.f-secure.com/enu/home/ols.shtmlhttp://onlinescan.avast.com/http://ca.com/us/securityadvisor/virusinfo/scan.aspxhttp://www.eset.com/onlinescan/http://www.kaspersky.com/virusscanner
Scan Only - no removal
<links compiled on 02/14/2008>