Raspbmc overclocks itself by default to safe levels
Q: Does Raspbmc void the warranty by overclocking?
A: NO. Raspbmc overclocks your Raspberry Pi to a safe clock frequency which has been verified by Dom who works on the Raspberry Pi firmware himself. Furthermore, a change to clock frequency does not blow the OTP. It is overvolting that does this. If you do not wish your Pi to be overclocked, just edit config.txt which is located in /boot.
I've been running it fine for a while now (bar a few buffering issues due to mounting an external SMB NAS drive)
First thing I would check before playing around with the overclock settings is the power supply as these are the most likely cause of instability, good tips can be found from here:
http://www.raspbmc.com/wiki/user/troubl ... tutorials/
Next try a different SD card as many types of card are not fully compatible with the Pi