Boot using the Windows XP installation disk, press R. Then type the following commands:
FIXMBR
FIXBOOT <---at this point, it may already work; if not try:
BOOTCFG /rebuild
cd\
copy E:\i386\NTLDR
ATTRIB -arsh ntldr
copy e:\i386\NTDETECT.COM
ATTRIB -arsh ntdetect.com
E: should be the location of your CDRom where you have the Windows install disk located.
and check if those files are in the root : NTLDR boot.ini ntdetect.com
Contents of the boot.ini file :
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect