HijackThis should be used as a last resort in my opinion. If you keep Spybot and Ad-aware updated, they will find pretty much everything out there.
It's an entirely different type of malware though.
Say a hacker successfuly hijacks your web browser and manages to get svchost.exe installed through an old Java platform that was unsecured and into your windows directory.. You wouldn't know that the real svchost.exe resides in the windows/system32 folder.
Someone who can interpret a log will spot those and remove them. Pacman's Startup List is a great resource to check suspect files, but last I checked it was down for maintanance.
Spyware is a completely different type of malware. Usually low-threat data miners.