To expand on what was stated above, if you perform an “overinstall” (i.e. install Vista on top of XP), you will most likely experience system instability, driver conflicts or other “weird” hard to explain issues. While I’ve not yet had the pleasure of dealing with an in place XP to Vista upgrade, every other instance of an in place upgrade (other than those using the same OS before and after) has given less than satisfactory results.

I agree 100% with what labnuke99 says above… If you want to create a dual boot system, the easiest way to do what you are asking would be to run Vista in a virtual machine instance (using the free VM Ware Player, for example). You would need both a valid XP and a valid Vista license, but I’m not sure if the XP license allows you to run Vista as a guest OS (can anyone clarify?)

The next possibility would be to obtain a copy of software which could dynamically repartition your drive (the old PQ Magic product, now owned by Symantec and called Norton Partition Magic) will allow you to do this. There may be other products, but I’m not aware of them

If you have the ability to add a hard drive to your system, you could install Vista on a new drive without repartitioning.