Need To Start Up Twice

This particular problem is starting up the computer. I need to start it twice before it goes (loads & runs windows (XP/SP3) properly).

Quite a while back this PC mostly wouldn't start at all; it was very temperamental and seemed to be getting worse. I then started swapping the graphics card around for some different ones and with these I thought I'd need a higher power PSU so I changed my Octigen 300W one for an OCZ 500W StealthXStream. For a few months maybe more the computer performed nearly perfectly. Then, recently, the current problem started.
1st power up from cold - the computer loads windows and gets to the log in screen. I type a varying number of characters of my password, sometimes 1 or 2, sometimes all after which my computer locks up - no response to keyboard, mouse pointer is fixed. This persists for as lng as I care to leave it. The computer is still receiving power. The only option I can think of is to remove power; the on/off button still works
(press for 5s).
2nd power up (shortly after - I haven't tried leaving it after first power on and shut the system down properly). Starts up ok. I always get 'you have modified start up programs' menus which I 'OK' and 'Cancel' respectively.

So what's the problem ? (I know, upgrade O/S etc)

Dumb_Question
3.February.2016
Compaq Presario S5160UK DT261A under XP/SP3
Processor - Celeron 2.7 GHz
Motherboard - MSI MS-6577 v2.1
RAM - 1GB +1GB DDR PC2700
PSU - OCZ 500W StealthXStream {upgraded from Octigen 300W model 10270PSOTG ('upgraded' from original

Bestec 250W PSU [in 2011?])}
BFG Overclocked GeForce 6800GT graphics card in AGP slot.

 

Overclocking your 6800 can easily surpass your power supply. Return it to stock clocking first.
Your power supply can only supply 18A on the 12V rail and an OC 6800 can pull 20A. Also consider you may have permanently damaged your power supply so measure the voltage on the 12V rail under load if you can.
After that try for Safe Mode again.

 

Thanks for tip, DOA.
How on earth did you find out the current draw of my card ? I've looked and looked for that sort of information. The card is factory overclocked out of the box @370MHz (+6% apparently - I believe stock frequency is 350MHz). I've not changed anything (don't know how ! I can't understand the Nvidia control panel so I don't open it)and I'm not running any graphics intensive programs (the most I use is MS paint and the odd video). Might even try to make a current measurement by cutting the 12V line on a molex extension and putting a suitable ammeter in there. This will not tell me about transients though.
Is it worth trying an XFX TS450 psu with its 44A 'easyrail' single 12V rail technology ?
Can the voltage levels be displayed by one of many software applications ?
I'll try safe mode at some point

Dumb_Question
5.February.2016

 

I Googled "BFG Overclocked GeForce 6800GT current draw" and started reading. 18A to 20A on overclocked cards came up repeatedly.
But only in gaming. If you have never stressed the card, your power should be fine.

I would toss in the power supply as a check.
With a number of failed starts your Windows install might be corrupt, or it might start, fail on a driver then restart without the driver and not get too far. Solution is the same, fresh install. You really want to use at least Win 7.

 

Can I check my windows install for corruption using some internet resource ?
I don't want to do a factory restore as then I'll need to do a lot of work reinstalling my software and worse all the MS updates that will be difficult to find for XP. Windows 7 is not worth it on this machine, e.g., max memory is 2GB and on those computers with W7 just booted state uses more than that. However I take your points.

Dumb_Question
6.February.2016