You are best to slipstream them alongside Service Pack 3 to your XP Reinstallation CD using Lite full instructions are given in Windows Reinstallation Guide: http://philipyip.wordpress.com/dell-community-forums/
Follow page 334 in particular. Note the link on page 335 to Intel Download Centre is broken and I need to update it however for a XPS 400 (it is assumed you have one because of the title of this thread if not confirm your model) use the following file for the SATA Drivers http://ftp.dell.com/SATA/R130118.EXE and ensure you slipstream the extracted contents (page 339). Once you have made the new CD follow A Clean Install of Windows XP (page 208).
Note however Windows XP is reaching Windows XP SP3 and Office 2003 - End of Life April 2014 and Microsoft will no longer update it making it very insecure. The XPS 400 also known as the Dimension 9150 will run Windows 7 64 bit so long as 4 GB of RAM is installed. I recommend you ditch Windows XP and instead install Windows 7 but this requires purchase of an upgrade or OEM license. Dell don't support this OS on this model and don't provide Drivers on their Drivers and Downloads page for Windows 7 for this model but here is an Unofficial Driver Set: http://en.community.dell.com/support-forums/software-os/w/microsoft_os/4625.dimension-9150-windows-7-64-bit.aspx
You can follow the instructions to install Windows 7 64 bit using a Digital River .iso without a product key to evaluate your systems performance on Windows 7 for up to 30 days before deciding whether its worth the purchase.
