At what stage is it rebooting? Does it restart immediately after the intel start screen? Go into BIOS settings and disable the logo at bootup and you will get the details of BIOS booting... Might have some information there...
Also, if you opened your PC for cleaning, open it again to check if all cables are connected properly, RAM is seated properly, etc... Also, if you start the computer with case open, you might (in some cases only) see something that could help identify the problem...
Is there any beep sound when booting? If yes, check the number of beeps and sequence of long and short beeps. Along with the BIOS manufacturer info, it is possible to find what the beeps mean and rectify...
Arun