Mac computer keeps crashing and won't open folders.. Any help?

I have a iMac, not had it long, at most, two or three years, though i have downloaded a lot of music onto it. a while ago it completely crashed and i had to reset the hardware, since then its performance has declined, and now, simple tasks such as loading two sites at once or playing music from itunes while using the internet causes it to pause for a minute or so showing the spinning wait cursor. Most annoyingly, when i try to open a folder from the desktop or something like that, it stops, with the wait cursor, then all the folders dissappear for a second or two, before reappearing, without opening the folder i clicked on. Can anyone offer me advice?