I have just got a Gigabyte GA-7DXR+ mobo with an AMD Athlon 1.21ghz cpu. When I'm playing games my PC just restarts itself. I told my system to give me error messages, which it did, 2 different ones, but now it just restarts. I thought it may be the CPU overclocked, as the guy who gave it me said something about this. How can I check if it is overclocked and how can I set it back to what it should be?

Clear the CMOS by removing the mains lead and CMOS Battery for 15 minutes...... It may also be worth considering removing the CPU HSF and cleaning/reapplying thermal paste.

There's no reason to clear CMOS. Run CPU-Z to check the clock settings for your CPU: http://www.cpuid.com/cpuz.php Also, check the CPU temp in the BIOS. Usually under "PC Health" or something similar. Another possibility is you have a weak PSU & it's buckling under load. "I told my system to give me error messages, which it did, 2 different ones" It would help if you said what those error messages were.