Ok, we know it's a 2.5 inch drive. If you only have XP PRO (the CD) then we must limit our drive size to 127GB or less (see http://www.48bitlba.com)
http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c01129734/c01129734.pdf and others seem to note this is an IDE interface which are rare to never found at best buy.
Try Newegg.com or geeks.com
If you have XP PRO SP2 as your CD then you can use a larger drive.