I think you'll need to contact Dell and get a downgrade CD from them, as the keycodes for the installation will be different between XP and the Vista licensing you have. Small fee for this, but not much.
Or, if you've got an available unused copy of XP, installing it should take care of just about everything. If you need specific drivers for the Inspiron 1525, here's their support site, where you can download all of them -
http://support.dell.com/support/downloads/driverslist.aspx?os=WW1&osl=EN&catid=-1&impid=-1&servicetag=&SystemID=INS_PNT_PM_1525&hidos=WLH&hidlang=en&TabIndex=