SONY VAIO PCG-7G1M Wont boot

Question

Hi Ive got a laptop and its out of warranty, a few months ago it decided to pack up and when it booted it said unable to find operating system and figured it out to be the hard drive had gone so I bought a new HDD, I then re-installed XP and booted into windows but a problem that was occurring before the hard drive died is happening again now.When I turn on the laptop the XP loading screen shows sometimes and then just turns off so I restart and If i put a little pressure above the numbers at the top of the keyboard it boots perfectly into windows but then I get a message saying new hardware found ACPI uniprocessor software installed restart the computer. So I restart and it keeps doing this over and over again. I cant remember the last time I booted into windows properly.ThanksPaul