|The .pst file for Outlook is meant for ease of backing up and can only really be recovered by importing it into another copy of Outlook - it stores your messages, contacts, calendars, settings etc. There are programs (mainly paid-for) that can extract the data from the file but I cannot recommend a particular one. The idea is that you can simply set your backup to save this one file and that's it, job done. There isn't an easy way to export and save your messages, contacts etc from Outlook automatically.|
From experience, if memtest doesn't find an error with a chip in the first couple of passes then it's unlikely to do so over a longer period unless you have heat issues with a system, but that would not normally cause boot problems, more likely to cause freezing issues.
Due to the age of the computer I would be more inclined to suspect the harddrive, and this is more likely to cause boot issues - use the manufacturer's own diagnostic tool to test it.
Finally, it is not unusual for external harddrives to cause boot problems - often it's simply a case of using a different USB port, but it does indicate a potential failure of some sort, ranging from a motherboard issue to even a power supply failing.
"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd