What I would suggest you do is to Backup your Data Files.
For this I would suggest using a External Hard Drive Enclosure. This will allow you to keep your Important Data safe and Separated from the main system in case of problems. The cost for this kind of Backup System has come down a lot. I would suggest a 500 Gig capacity for your needs. Since you will only be Backing Up Data Files Only this should be sufficient to do that for you. Follow the next two links to see examples of the price you will be looking at.
I suggest going to Microsofts Backup Tutorial and follow the steps to do a Backup to a External Drive and also it will guide you on how to do a Scheduled Backup even at night.
Another way you can do a Backup is by using DVD’s. This method is just as effective and can be done using the same steps as above.
After the above is done then go on to the next step.
Repair Installation Instructions
· Insert your Windows XP CD in your CD-ROM and then Restart your computer.
· At the first post screen start tapping the Del key to enter System Bios
· Find the Advance Options or Boot Sequence screen.
· You will need to change the boot sequence so that the CDrom is first and the HD is second.
· Press F10 to Save and Exit and then press Enter to accept.
· The system will now reboot
· When the "Press any key to boot from CD" message is displayed on your screen, press a key to start your computer from the Windows XP CD.
· When you see the following message displayed on the Welcome to Setup screen, press ENTER
· To setup Windows XP now, press ENTER.
· At this point an option to press R to enter the Recovery Console is displayed. DO NOT SELECT THIS OPTION.
· On the Windows XP Licensing Agreement screen, press F8 to agree to the license agreement.
· Make sure that your current installation of Windows XP is selected and highlighted in the box, and then press the R key to repair Windows XP.
· Follow the instructions on the screen to complete Setup.
You can also go HERE
to read a step by step tutorial with screenshots.