Windows 7 x64 random reboot

Hey everyone. Recently my PC hardware was breaking down when I was running Windows XP Professional, so I decided to upgrade all my hardware since it's been 4years. Got a new ASUS motherboard, a new i5 2.88ghz processor, new RAM, new monitor, new gpu, new heatsink etc. I also got Windows 7 Home Premium 64bit OEM through newegg.com. I'm not a huge tech myself so I payed some local tech to install it all. There is only one problem now.

I can run games that are intense on the CPU and GPU and it won't reboot and I can do normal stuff in OS that won't make it reboot. I notice it reboots everytime I try to install a new program of some sorts. It doesn't give me an error msg. It just restarts and when it starts windows it states that windows did not close properly and asks me if I want to start it in safe mode.

I've checked the temperatures of the cpu and gpu and motherboard and it's all normal so I have no idea what is wrong. I also got a new seagate 800gb hard drive when I got new hardware and the tech reformatted the drive and installed windows 7 on it. I really would love to get to the bottom of this without reinstalling windows or whatever. Any suggestions to find out what kind of error I'm getting and how to fix it? I greatly appreciate any help. Thanks so much in advance.

You think It's bad even though the restarts don't happen on hours of graphic intense games? I'll get a new one but only If I'm positive It'll fix the problems. If so, what kind of power supply is compatible with my computer + is affordable?

Thanks for the quick replies and help. So far the random rebooting has stopped, but if it comes back I'll highly consider a new PSU. I live on the east coast USA and would like to keep the spending limit to around $100 give or take some. So any PSU should physically fit where my current one is? Cool.

Yeah, newegg.com is where I ordered all the new hardware such as the motherboard, cpu, etc. Actually I did a system restore and so far I haven't had a random reboot, even when installing new programs. Hopefully it stays that way, otherwise I'll llook at a new PSU. Thanks guys

Well, It's rebooting again. I installed a lot of programs recently, and they all work fine, but I just tried to uninstall Quicktime to rollback to an older version, and after windows tries to load the uninstall program for quicktime, it just shuts off, and reboots. I highly doubt it's something to do with the power supply if its happening when I uninstall stuff and everything else runs fine. Any more troubleshoot tips?