This assumes that you have already checked your floppy for a non system disk.
1. Start your computer with a Windows XP CD-ROM.
2. At the Setup screen, press R for repair.
3. Opt to use the Recovery Console (press c).
4. Type 1, and then press Enter (if you have more than one windows install then choose the number of the install you want to repair).
5. Type your Administrator password, (the one that you setup when you were installing the OS) and then press ENTER. If you do not know what it is try leaving it blank. If that does not work, find out what it is from the person that did the install.
6. Type map, and press Enter. This should tell you the drive letters assigned to the drives and what type of drives they are. this is very useful if you have multiple drives. This is for reference only.
7. Type: copy "drive":\i386\ntldr c:\ ("drive" will be the drive letter that corresponds to the CD-Rom Drive your XP CD is in. Refer to step 6.) Press Enter.
Type: copy "drive":\i386\ntdetect.com c:\
Type y If you are prompted to overwrite this file, then press Enter.
8.You can do this to check your new boot ini file if you wish but this step can be skipped if you are confident that all is good.
Type: type c:\Boot.ini
You should see something like this:
[boot loader] timeout=30 default=multi(0)disk(0)rdisk(0)partition(1)\WINDOW S [operating systems] multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Micro soft Windows XP Professional" /fastdetect.