I had the same problem in my company, in my case it was a spyware/adware program.  Scan it with adaware http://www.lavasoftusa.com/software/adaware/\nand MS antispyware\nhttp://www.microsoft.com/athome/security/spyware/software/default.mspx\nthese to program will find it and remove it, if you have never ran an antispy ware program in this laptop you will probably find a lot of them, once you scan you pc and it asks what actions to take after the scan make sure you follow the software's recommendation.  If spyware/adware is not the problem your pc is probably overheating, a clear indication is  when the fan does not work.
Why does a laptop keep shutting down?