Hundreds of times. It is rediculously easy. Only thing though, is if you have them install it, they install it with some cool little extras and preload all of your drivers and stuff. Also if you have an OS problem and YOU installed it, then it isn't under warranty. If you have ever installed Windows XP on a pc, it will be exactly the same on the CX7. (except maybe a tad faster )
FYI: Make sure you have XP Pro with SP2. SP1 and previous did not have SATA native support and getting the SATA drives in these things to work can be a bit of a pain (unless you've done it a few hundred times).