Google it.
There's some very common threat assessment webpages out there. You can just put "csrss.exe" into the google box and hit ENTER, and the first 5 should have general info on the process.
You have to make up your own mind what you want and don't want. In the msconfig screen you can put the entry there ("GWHotkey" for example) and you should be able to get some info on it.
Generally in the msconfig screen you can see where it's located for a directory name, like if it has ...\Symantec\... in it, it usually relates to antivirus.
It's a bit of a detective game. Do some searching, check it with what software you know you have installed, and decide what you personally want to disable or leave enabled.
I'm down to about 24 processes when idling.
EDIT: Having said that, keep in mind some anti-virus software runs 10 processes at a time, so anywhere around 35 or so would be "decent".