When you say frequently rebooting; Is it randomly rebooting without warning? Do you get a BSOD (Blue Screen of Death) error message? Or does the PC just reboot without any error's other than windows was not shutdown properly?
MOST random reboot issues are caused by Power issues;
-Failing Power Supply
-Blown Capacitors on the motherboard
-Insufficient power to run the system.
These would be a good place to start to troubleshoot if you do not get a Blue Screen Error Message.
-Failing Hard Drive can cause this issue as well but is usually acompanied by a Blue Screen Error of sorts.
-Failing RAM again usually accompanied by a Blue Screen Error.
You can test for failing hardware, by using Hiren's Boot Disk for diagnostic tests on your Hard Drive or Ram.
Hiren's Boot CD 15.2 - http://www.hirensbootcd.org/files/Hirens.BootCD.15.2.zip
You will want to run memtest X86 to test the Ram or Hitachi's Drive Fitness Test or SeaTools to test your hard drive.
This BootCD is not for the typical end user whom may not know there way around the features and options this Tech CD provides.
You will want to burn the Hirens Boot.ISO file to a CD (Burn Image Option) using any of your favorite flavors of burn software.
Hope this helps.
You can also download these diagnostic test individually such as memtest x86 and Hitachi's DFT or SeaTools independently from the Boot CD just by doing a google search. This might be the easier way to perform these tests for people who aren't very savvy with boot CD's.