PC Shuts down after 30 seconds

Hi everyone!!

Firstly, this isn't a homebuilt system, it's actually a regular PC that I bought... I don't know much about computers and their detailed specs but i have this PC which only a few months old.

It's an:
Intel Pentium 4 CPU 3.20 GHZ

Basically, when I switch it on, it goes through the normal star-up process and then asks me for my password as per normal. Once I type it in, it looks like its working but in truth it immediately shuts down and auto-restarts.

I have tried to start it in different manners: Start Windows As Normal, Start Windows with previous settings that work and it didn't work.

However, it loads under safe mode and seems to be remaining on for longer... I need to see if it continues to stay on or switches itself off but so far it seems fine.

Could this mean I have a virus which is why it opens in safe mode but not in other modes? If that is the case, then what can I do to fix this?

I love this computer, I just hope someone can help me... I'm good at following instructions if they are clear, i'll do anything to fix this!!!

Thanks guys!

hello! firstly thanks for ur help... no this comp is about 1 year old... and it's been on for over half an hour in safe mode so it's remaining on... does tat mean it's prolly a virus?
a the beginning of this problem, the comp was switching off randomly after about 5 minutes, now it's more or less 45 seconds...