Computer shuts down during downloading

[Ah88]

Hello

I would like some advice on what to do regarding the problem with my computer

My computer wont allow me to download large files,

When I try to download large files, the dialogue box comes up with the options: Run, Save or Cancel. But when I click Save it starts to download and then it reaches to about 5%... then this is when the computer shuts down and restarts.

I have tried to download anti-vrius software but the same problem happens
I dont think the problem is because of low memory space,
I have also used system restore to restore the computer to an earlier date


I would really appreciate it if someone could help me on this,

Thank you very much