weird windows XP pro (media center) problem

[transviruses]

couple months ago my family bought a HP Pavilion 061 pre-built PC from future shop. recently when i choose to "shut down" it restarts by it self instead of shutting down like it should do. this happenes quite frequently.
Any ideas what should i do to fix the problem? thx
btw when ever i start using Quick Time player to watch HD trailers it restartsby it self... thx

pc specs
CPU: Intel Pentium D 2.80GHz (2cpus)
OS: Windows XP Professional (media center edition)
BIOS: Phoenix Award BIOS v6.00PG
RAM: 2GB
Direct X version: 9.0c
Video Card: NVIDIA GeForce 7300LE (256MB)

 

[Sjbrand99]

Have you recently installed any new software? If so then try reinstalling it.
Have you recently installed any new hardware? If so then check the drivers.
Always take these steps first for problems like this.

Hope this helps!

 

[transviruses]

i didnt install any new hardware
yes i have installed many softwares because its a new system, so im gonna try to reinstall everything. thx for helping