this is patent bollocks, its less secure because of the ways in which it allows 3rd party software access to system resources. yes they've got better, but still have fundemental flaws such as running privileged user as a default, allowing user applications tight integrated to OS internals...
not really, you may have a knackered drive and as noted may be about to fail completely.
run cmd (start, programs, Accessories, command prompt) as administrator. run "chkdsk" and see what it says about bad sectors. the fact that a repair fixed the stalling bootup would have me ordering a new...
accountancy, business software, you'll probably want Windows. if doing website development work, probably a Mac is better.
on the other hand, with the online apps these days you could probably do alot of business related work on an iPad or other tablet.
so consider what software you need...