Read the "read this first" sticky post that suggests a lot of things to do. One thin in particular, all the plugins *have* to be of a compatible version, even if you never use them. Pro Tools loads everything on startup wether you use them or not, and an incompatible version will mess something up.
IK Multimedia plugins, like AmpliTube DUO, Ampeg etc. seem to cause a lot of 6101-errors even if they claim to be compatible; Removing them made all the difference for me. Try the SansAmp plugin instead (Eleven is very very resource-hungry).
Look for a "support folder" (a file package) somewhere in the support pages, it contains a small program to help you list the plug-in versions. Run it while Pro Tools is running, and make sure the output file does *not* exist before you run it (a text file at the desktop).
Using the internal disk should work OK for small projects, if everything else is OK, but an external drive is recommended anyway.
As for other things you could do: make sure you've done every step in
http://duc.digidesign.com/showthread.php?t=193636
BR, Robert