Everytime i press the on button on my computer it switches on until it gets to the 'DELL' screen then reboots again and it keeps going that until i press the power button to switch it off.

How do i fix this?

I'm on my friends laptop at the moment

Answer:
Sounds like its overheating to me, try getting some canned air and cleaning out the fans and such.
Answer:
yeah, it could be overheating but it could also be becuase:

1. CD in drive

2. flash drive

3. Bios issue

4. failing power supply

if it gets past POST, post operating system test, (fancy techie word for hardware checkup) then you got a VIRUS!!!!!!! ouch, if so i advocate you get a distro of linux that can boot off the CD and get to work in fixing the viruses.

Answer:
A friend of mine had that last week. Turned out she had 8 viruses that her virus software didn't catch. She had to take the PC to a professional.

-MM