You may be able to use a Windows XP bootable Floppy Disk
to boot from a diskette instead of your hard drive. If your hard drive's boot sector or Windows' basic boot files have been corrupted, this disk will circumvent the problem and boot you into Windows. If you don't have an emergency boot floppy, you may be able to use one created on another PC running Windows XP but there's no guarantee that it will boot your machine.
Another option is to create a Bootable CD:Bart's way to create bootable CD-RomsCreating a bootable Windows XP CD with Nero Burning ROMCreating bootable Windows 2000/XP/2003 Disc with Nero 6
Some types of malware can result in a system so badly damaged that a Repair Install will NOT help!
. Reinstalling Windows without first wiping the entire hard drive with a repartition and/or format will not remove the infection. The reinstall will only overwrite the Windows files. Any malware on the system will still be there afterwards and may still be responsible for your inability to boot properly. Starting over, reformatting the drive and performing a clean install removes everything.
"How to reformat and reinstall Windows XP - Method #1
"How to reformat and reinstall Windows XP - Method #2
These links include detailed step by step instructions:
"Clean Install Windows XP
"XP Clean Install Interactive Setup
"Clean Install Procedure with Illustrative Screen Captures
Reformatting a hard disk deletes all data. Since you cannot access your computer one option is to is to remove your hard drive, install a new one, and make the old one a "slave". That way you should be able to access it and salvage some of your data files.
"How to Change the Master/Slave Designation on a Hard Drive
"Data Recovery Tutorial: How to Slave a Hard Drive
You can start a new topic in the Hardware forum
if you need assistance with that. Hardware is not my area of expertise but we have other members here who can walk you through step by step.