Dell Inspiron 5150 will only go into windows safe mode

Hi this is a real brain teaser and I have run out of Ideas

The Inspiron 5150 seems to work perfectly except it will only go into Windows safe mode.

I have run the Dell diagnostics check and it tells me everything is working OK

Also on installing win XP Pro SP 3 it gets as far as the devices section and the time bar freezez on every try.

Taking  the drive out into another 5150 the HDD boots up into Windows XP Pro Dual Boot and works fine right away and I have cross checked the following with another Inspiron 5150 and they all work fine

RAM
HDD
Windows
CD RW
Re flashed Bios to A38 as listed on the site as it was A34 and I think someone may have fiddled with it before as when I tried to load the Bios .exe file in windows safe mode it didnt allow me as it said it wasnt connected to a battery or adapter which it was and it referred me to Sam Wong/Anthony Chi in London SW!!!!

Question:- I havn't substituted a CPU. Are the clock rates pre set so the motherboard is stuck with the listed CPU speed 2.8 or can another speed be inserted eg 3.06, 2.6 etc

It sees the charger fine
It charges the battery fine
USB ok
If anyone has got any ideas I would be pleased to try them its a real brain teaser and any comments on the cpu would help.

Also I have serviced the laptop, cleaned the cooling fan/fins (it was filthy) put on new heat conductive paste between the cpu and copper heat sink pad but nothing prevewnts the fault so far, very frustrating.

Kevin