There is a IEEE 1394 (firewire) driver patch available for Windows XP SP2. Even if you have automatic updates turned on, you will still need to manually download and intall the update
Please go to www.microsoft.com and search for "SP2 1394". The top search result links to an article explaining that some firewire 400 devices might not perform as expected without the required update. Download and install the update (KB885222) and then restart your PC. You should now be able to re-install the Saffire.
If you still experience problems, then make sure your computer is optimised for audio as explained in this article;