Delete any and all shortcuts in the fonts folder (C:\WINDOWS\Fonts), and replace them with the actual font file. By expanding the following Key in the Registry (Start | Run | Type...regedit, and click OK)...
HKEY_LOCAL_MACHINE | SOFTWARE | Microsoft | WindowsNT | CurrentVersion | Fonts
...you can also double check the path of each font and look for any suspicious paths (check all the path's listed in the RH pane of the above listed Key).
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 fonts folder. Once this program is uninstalled, the shortcut may now point to a font file that no longer exists. XP in turn reacts by turning all unspecified fonts into italics upon next reboot.