iMac freezes, no choice but to hard shutdown, turn back on, white screen with folder and question mark appears

Hi,

so my iMac (it's not the most recent iMac if it makes any difference, and I'm running mountain lion 10.8.3) often freezes and I do a hard shutdown as I cannot force quit the applications or click on the apple in the top left to shut it down. When I come to turn it back on, I'm greeted with a white screen, in the middle of which is the dreaded folder with the flashing question mark in it. I've noticed that just before a freeze occurs, it starts running slowly and if I try going onto Facebook for example, it says that it cannot connect to facebook.com despite the internet connection being fine as I can browse things on Google. I have also noticed that before a freeze, in Finder, in All My Files, no files show. I went through this http://support.apple.com/kb/TS1440 and the problem still persists. Sometimes it does load fine if I shut it down again but when I eventually open up safari, if I had been on youtube for example, it will reload the page from a few videos before and not the one I was on when it actually froze.

Any help would be hugely appreciated,

Thanks

Side note: When it says I must choose a community in order to post this, it only gives me a choice of 3 which are Mac OS X v10.4 Tiger, v10.5 Leopard, v10.6 Snow leopard, so apologies if this is posted in the wrong place - is there any way to get it moved to the correct place?