What error message are displayed? It could be a number of things. But you may have to boot with an emergency disk. Hopefully you have made a good emergency boot disk. You can always make a windows startup disk by creating one from another computer running Windows 98 or Me. Perform the following if your computer won't bot up at all.
Put your boot floppy disk in the floppy drive and turn on the PC. On some computers, you may have to access the bios and select the Boot priority to your A: drive. Save any changes and select "Start Computer without CDROM support" and press Enter. Once you are at the A: prompt,type dir c: and press enter.
To correct the problem of your computer not booting up, type in "fdisk /mbr" and press Enter to restore your master boot record.Type "Scandisk C:" to check the hard drive for errors that have occurred. You can also type "Sys C:" to hopefully restore files needed to boot up your computer.
If the above procedures fail to repair your computer,you can repeat the first part of step one above and select "Start Computer With CDROM Support" re-install Windows. Good luck with that. A lot of times, PC won't fully boot because of missing "ntldr,ntdetect.com and boot.ini" files. You need a Windows XP boot disk for that. This is how you can creat your own XP boot disk: make it off of a friends computer or something.
Place a formatted blank diskette in the A: drive, open Windows Explorer to the C:,select Tools, Folder Options, show Hidden files and folders,then View Tab. Now you uncheck "Hide Protected Operating System Files (recommended). You will see a warning and click Yes and click OK. Copy the files ntldr,ntdetect.com and boot.ini onto the disk.Remove the disk and label it the Windows XP Boot Disk.
After making your boot disk,recheck "Hide protected operating system files and folders (recommended)" in the Folder Options dialog box. To use the disk when Windows XP won't boot,place the disk in the drive, and unto re-booting,the computer, Windows simply bypass the basic boot files on the hard drive and continue to boot up.