|If you have a Windows XP CD (not an oem recovery cd) you could try a repair install. This will not delete your own documents or pictures, but it's always wise to have backups just in case - obviously in situations like yours (no Windows) backups should already have been made.|
How to perform a repair install:
If you don't have a Windows XP CD, you'll have to perform a factory-default recovery instead. This is accomplished by booting from the Recovery CD if one exists, or by pressing Ctrl + F11 keys together when you see the 'Dell' splash-screen during the boot process..
If you've been sensible and have already created backups of all your own files, a re-install or recovery need never be a "last resort" measure.