There is no such thing.
I have installed many software on the family computer (windows XP, and it is getting very slow and sometimes crashes.
Crashes are mainly due to either faulty hardware or incompatible drivers. Sometimes, driver updates (or "downdates") is helpful.
Okay, poor applications might crash also. In that case, get rid of these using the "uninstall" procedure.
Additionally, malware can be another reason for system instabilities.
Will a registry scanner work making my computer better and decrease the chance of errors?
What you can do is checking the automatically loaded stuff. Start -> Run -> msconfig.