Lockups and freezes can be a pain to diagnose. If you want to definitely rule out overheating, just take the side off the case and put a box fan/large desktop fan blowing right into it onto the components (don't put the fan itself too close to the case or hard drives though, as they do have a bit of a magnetic field with the bigger motors running).
Also make sure you have updated drivers for everything you can find for major system components, especially video, sound, storage, etc., as well as the most recent BIOS from the motherboard/system manufacturer. Many times these can help with hardware related lockups.
As for software related lockups, maybe a reboot into safemode with networking may help. If it doesn't lock up then, possibly some software running on the system is giving you problems or conflicting with something else.
Do make sure you aren't running multiple security products with active scanning at the same time, like 2 or 3 antivirus that are all scanning the same files that are opened simultaneously.
A chkdsk on the hard drive wouldn't be a bad idea either to check for bad sectors and corrupt filestores.
Even SFC /SCANNOW could help if there is a corrupted Windows file.
Did you clean the dust out yourself, or have some one else do it? Maybe they missed something. With the system turned off, grab a can of compressed air, hold it upright so it doesn't spray liquid out, and blow out every fan, crevice, vent and hold you can find. Make sure to especially be vigilant to get every fan and vent you have on your video card so nothing is missed.
Does it lock up randomly, no matter what you are doing.. checking email, in a Word Document, playing a video game, browsing the internet, watching videos on the internet?
Does it lock up when the system is idle and you're not around actively doing anything?
Does it only happen, or happen more when you do specific things, working with specific programs, play specific games, etc.?
Is this a new computer? Did it start doing it right when you got it/built it? Or is it an older computer that worked perfectly fine for a while, but then just recently in the last year started this?
Have you looked in Windows Even Viewer > Windows Logs > Application / System for any warnings or errors that may be recorded before the system freezes. Best time to look is right after reboot.
You can even clear and save the existing log files and start fresh so you can see what is now happening and sort them after you are forced to reboot.
I would also use a program to check the voltage output of the PSU, if voltages fluctuate greatly from a bad or cheap power supply, system freezes and lockups are a typical sign.
Sorry for posting to such an old thing, but these standard troubleshooting steps might be able to help someone else who experiences similar issues and doesn't know where to start.