You can download the OS installation utility from the Drivers and Downloads page of support.dell.com for your system. Here is the latest version for the 2650:
www.dell.com/.../poweredge-xeo2650
The above utility will ONLY work for Server 2003 (WITH SP2), 2008x86, and a small group of supported linux distros (it will NOT work to help you install XP).
XP is not supported on ANY Dell servers. That does not mean it won't work, but you must consider the possibility that, try as you might, you may never get it installed or running correctly. Many of the devices were made specifically for enterprise/server use and thus the manufacturers of those devices never wrote drivers for client OS's (XP). Your best bet will probably be to use Server 2003 drivers.