Event Log, Administrative Tools > Services (Disable all unnecessary services). If you're running a server version of Windows, remove your server roles and then re-add them, check things like WMI for corruption, clear temp directories and Temporary Internet Files.

Also, check your hardware. Is your RAM seated properly? Is your CPU overheating? (AMD only). Change your nVidia and chipset drivers (usual causes of STOP errors), scandisk & defrag with DiskKeeper or OODefrag, and make sure you're not running something like ABC/BitTorrent, which is known to stress system components and cause BSODs and reboots.

Another often-overlooked issue are your NIC drivers; most people don't think that installing appropriate drivers for your network adapters is a necessary part of system maintenance, but if you're doing ANYTHING which uses your LAN or WAN, upgrading your network card drivers to the latest version is a worthwhile task.

Last but not least, run AV and Lavasoft Ad-Aware and see if anything comes up. If you're still left having issues, change your (most likely under-powered) PSU, and then finally reinstall.