Please be careful with Windows XP. Microsoft are no longer officially supporting it, so it will be unpatched against any new security vulnerabilities that appear. If you are not deleting or moving the _data folders of the projects, a virus may be doing so.
Make sure you have XP Service Pack 3. You can still download Service Pack 3 from Microsoft if necessary: http://support.microsoft.com/kb/322389/en-gb .
Ensure you have a good third-party anti-virus application. Be aware though that this will not give you complete protection against a previously unknown attack.
For some extra protection you can install http://technet.microsoft.com/en-us/security/jj653751 .
If it is not practicable for you to update to the much more secure Windows 7 or Windows 8, you can consider installing a Linux operating system instead. Most versions are free and all are very secure.
Gale