If you have 2 sticks of memory try reversing them (switch their places) and see if it still reboots, If you only have 1 stick, try setting it in the BIOS to a more conservative timing. (try a higher number on the CAS setting)
random reboots are consistantly a problem with Memory for me, if you reverse them and the computer doesn't reboot until much later on, it would indicate a bad chip, Alternatively you can download a program like Memtest86 (don't have link for it, but Google works

)
Best of luck.