McAfee update deletes part of Windows XP

McAfee Update Thinks Windows XP is a Virus | Maximum PC
Users of McAfee's corporate antivirus product found themselves wrestling
with some pretty serious
problems today. The most recent DAT update for the antivirus suite
caused the scanner to identify the benign Windows svchost.exe file as a
virus. The antivirus' course of action is clear; it deletes the file.
The result is a lot of crashed PCs and unhappy IT departments. This
isn't even the first
time McAfee has had an error like this.
...

Get the problem fixed, then remove McAfee and install AVG Free

Hold off on that McAfee patch!

McAfee AV Patch Leaves Some Computers Unbootable | Maximum PC

Oopsy-daisy! According to complaints on McAfee's message board, a mandatory service pack for the company's antivirus VSE 8.7 software has left some machines unbootable. The update, which was issued on May 27 and later pulled on June 2, was intended to squash minor security bugs, but also inadvertently flagged some Windows system files as malware.

This is another reason why I recommend NOT using one of the big commercial anti-virus programs.
I recommend using AVG Free.