Spy doctor is Good though in a recent head to head test it was claimed as being a bit over inquisitive finding "False Positives"

I use Ad aware SE ,Spywareblaster and Avast to keep trouble away from this computer(Am using others also) but for basic Antispyware protection and a Antivirus Protection, You can't go far wrong with this combination

and they are all Free and take up no space on the Harddrive
If you have Windows XP? Microsoft Have a very strong Antispyware program for free just set the timer and it runs itself

BTW Deleting spyware has no effect whatsoever on programs unless you have severe PopUps

or your Homepage has changed

PS Switching Windows Messenger off does ease the Popups. There is an addon for AdAware which you can do just that in 1 click

Otherwise ,the switch is quite deep in Windows
