When a rogue program decides to install its own version of system fonts, it may replace the original version of the font file with a shortcut in the font folder. Once this program is uninstalled, the shortcut may now point to a font file that no longer exists. Windows XP in turn reacts by turning all unspecified fonts into italics upon next reboot.
The simple way to fix this is to delete all shortcuts in the font folder and replace them with the actual font file. By going to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts], you can also double check the path of each font and look for any suspicious paths.