Please help - Repeating "Missing operating System" error

[roy34543]

Well im writing this on a laptop right now. But I have a much bigger desktop that keeps having this problem.
Its happened 3 or 4 times now and me and my family keep getting the computer "fixed"(at some kind of shop) But it keeps having the same error every several weeks or months.
Apart from that, if you shake the computer too hard (ie , knock back the keyboard tray or something) the computer freezes entirely. Im almost convinced that it must be a faulty hard drive thats the problem, but I cant confirm it. Can anyone shed some light on whats the most likely problem? Should I get a new hard drive? Im not a computer expert and ive never messed around inside a computer case before, so please dont ask me to look inside of it or something. .

Right now its being taken to be fixed again, so I couldnt anyway . But id really appreciate it if anyone could confirm that its almost certainly a faulty hard drive, or if it could be somethig else. And if so, whats the best course of action? Keep the same hard drive and just repair it again and again or get a new one? Please note the computer is using windows vista.

[Jaketh]

i used to have loose RAM that whenever i shook the case too much would cause a shutdown or bluescreen, make sure your RAM is secure

[Ishan]

Make sure you HDD is plugged in right and also follow Jake's lead.