What version of XP are you using? If you are using XP Pro you could use
gpedit.msc to open local Group Policy and go to user
configuration/administrative templates/Windows components/Windows Explorer
and make sure that the setting for prevent access to these drives is set to
disabled which may fix the problem but it still does not explain how it
happened in the first place which could be malware related if that setting
was undefined. It would be a good idea to check your computer for malware by
scanning for it using the latest virus definitions for the program you use.
Removing malware however does not usually undo restrictions that the malware
placed on your system. --- Steve
http://www.microsoft.com/athome/security/protect/windowsxpsp2/Default.mspx
--- Protect Your PC tip and links from Microsoft.