if you have a win9x/ME installation CD, evrything would be easy

- enter into DOS boot disk

- fdisk /mbr

for windows XP installation CD

- press 'F10' or 'R' at "welcome to setup" screen

After you start the Windows Recovery Console, you receive the following message:
Microsoft Windows(R) Recovery ConsoleThe Recovery Console provides system repair and recovery functionality.
Type EXIT to quit the Recovery Console and restart the computer.

1: C:WINDOWS

Which Windows Installation would you like to log on to
To cancel, press ENTER)?

After you enter the number for the appropriate Windows installation, Windows will then prompt you to enter the Administrator account password.

NOTE If you use an incorrect password three times, the Windows Recovery Console quits. Also, if the Security Accounts Manager (SAM) database is missing or damaged, you cannot use the Windows Recovery Console because you cannot have correct authentication.

Once you are at the Recovery Console type the following:

fixboot c:
fixmbr

If you were having issues getting into Windows before, you will have to rebuild your boot.ini by entering the following command.

bootcfg /rebuild