Make sure you can get XP drivers from the manufacturer before you try to install XP.
Depending on the system you may need a hard drive controller (SATA or RAID driver) when prompted from a XP install disc.

The difference between OEM and retail versions of Windows is the licensing and no support from MS. The OEM version must stay with the computer it was originally installed on. The retail version can be removed from the original computer and installed on another.

Here's a good rundown.
Should you purchase an OEM license version of XP? - Microsoft: Windows XP Pro FAQ - Tek-Tips

Also maybe try posting your issues with Vista in another post. I've been using Vista for quiet a while now and have no issues. The issues are usually with trying to run older software or hardware on Vista where the manufacturer hasn't updated their product.