I just need to know if you partitioned the drive and installed 2000 on one partition and XP Professional on another, or have you done as stated and upgrade 2000 to XP?
You should have edited the boot.ini file in this case to remove the reference to Windows 2000.
It could be that the ntldr,ntdetect.com or boot.ini files are corrupt.
If you put the XP CD into the CD-ROM drive it should boot. (?)
If you have a floppy with the ntldr, ntdetect.com and boot.ini files on it it should also boot.
If you can run a KNOPPIX CD from the CD-ROM drive together with this boot re-director bootdisk
BIOS BOOTDISK it will boot Knoppix without altering the BIOS boot sequence. If Knoppix can read the drive you can save any data by using the K3B burning program which comes with the Knoppix CD.
If you decide to re-install with an XP SP2 CD then you might have a problem deleting old NTFS partitions so you may need to use this
DELPART BOOTDISKThere is also the
BOOTINIEDIT bootdisk which will allow you to edit the boot.ini file on NTFS volumes.
It looks as if you'll need a boot.ini file that reads as follows...
[boot loader]
timeout=10
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
Alternatively, have you tried booting to the Recovery Console and typing - fixboot - or - fixmbr?
Take a look at - bootcfg /rebuild - and if you type - help - at the prompt you'll see a list of the commands available for use with the Recovery Console.