Re-ocurring spyware problem, running windows XP all flavors home,media edition and or pro!

As of late I have seen 10+ computers that have been infested with spyware and have shown a similar problem. It seems to be a result or symptom of certain spyware as of yet unidentified.

After cleaning with any given spyware program(usually Spybot S&D) at reboot I get a logon screen for administator and a password request!

Problem is there was never a password set and no password seems to fit!

This has happened a NUMBER of times and usually a full system wipe out is the only recourse. Occasionally, with no rhyme or reason as to why, the password will request will dissappear and you can boot and complete the reapur and removal of the associated spyware!

Right now for instance it is a Compaq, runing XP media edition, and at boot up it is asking for a logon for "Compaq Administrator" and a password for same. No other logon choices are available and the owner NEVER remembered setting a
password for the Compaq admin!

So a couple of questions . . .
1) How did it get to the point of asking for a password?

2) Is there a gerneric password (for this compaq let's say)?

3) Is there away around this without wiping out all data and doing a complete new install?

4) In safe mode is there a choice that would bypass this? (As I said before I thought I had found a choice that DID indeed avoid the logon/pw. For some reason when encountering the password request it will randomly go away! )

thanks