BleepingComputer.com: XP prob on startup!!

Hi all

Last month i had a problem that occurred out of the blue where my pc on start up didn't do the usual by going to the XP logo & then to the blue 'welcome' page, instead i got a black screen & the following message :-

"We appologise for the inconvenience, but Windows didn't start successfully.
A Hardware or Software change might have caused this

If your pc stopped responding, restarted automatically or was automatically shut down to protect your files & folders, chose 'last known configuration' to revert to most recent settings that worked.

If a previous startup attempt was interupted, due to power failure or because the power or reset button was pressed, or if your not sure what caused the problem, chose 'Start Windows as Normal'.

SAFE MODE
SAFE MODE WITH NETWORKING
LAST KNOWN CONFIGURATION PROMPT
START NORMALLY

Use up & down keys to highlight your choice.
Seconds to Windows start : 28 "


There is no rule as to when this happens other than it happens when the pc has been off over night. Whichever option i chose, it will just loop into the same page then for no reason it will boot normally to desktop after many attempts
.
This went away for a month & has just returned after i have downloaded a few Microsoft updates including service pack 3.

I have run Seagate Seatools for windows that has checked my disk out & that says all is ok.
I'm increasingly worried as i need my pc for studies & am quite inexperienced when it comes to backing things up etc.
I hope someone can help me.
I am wondering if it something that is running on startup. I have deleted SuperAntispyware as that kept not shutting down properly & i recall the initial prob starting when i 1st downoaded this!
btw, my pc is a Compaq Evo small form factor, i have no XP disk.

Thanks

wf