Yes it can be done.
If you do not want to re-install XP, you will need to use a partition utility such as Partition Magic to change the drive partitioning.
There are several TechRepublic articles on setting up dual boot systems with Linux and W2K.
The tips in these articles should apply to XP also.
Search the archive with the terms linux and dual boot in the search box.
Also, check out the dual boot section at www.system internals.com
And, search Microsoft on the terms boot.ini and bootcfg
From the results, you will get details on using and editing the boot.ini file.
You will need to manually edit boot.ini to set up your system for dual boot after the fact.
Also, you might want to subscribe to the Linux e-mail from TechRepublic.
For info on Linux;
www.linuxdoc.org
www.linuxdevices.com
www.linuxorbit.com
Chas