, First I want to say welcome to BC.
Now .. we'll need some more info to work with.
How much RAM does it have
How old is the PC
What Antivirus and spyware /malware programs do you have and when were they last run and updated
What firewallWhen it happens
Are you getting a blue screen
Do you get any messages
What are you doing .. program(s), gaming,photo,music etc..
Are you on/offline or both
You can run the Event viewer (information on errors and crashes) and post that info
BC tutorial How To Use the Event Viewer Applet
You may have a heat or damaged RAM issue
Before touching parts inside the PC touch your fingers to the metal chaasis of your case,to disharge ststic from your body.
Can you touch the fins on the CPU (the heatsync) and see if it's hot. Warm is OK. Hot is like an ouch or pullback.
Cleanout the inside of your PC of dust bunnies, clean all fan bades and the fins on CPU
You can also run a RAM check Memtest86
and post the results