What is causing my MacBook Pro not to let me log in or view my desktop?

For the past few days, I have been able to turn on my MacBook Pro, but everything runs incredibly slowly. I can log in, but it takes a long time for the computer even to register my typed password (I see the dots for the encrypted characters, and the rainbow circle processing icon appears, but then I get at most to a gray screen, or a screen with my desktop wallpaper but no dock and no program or file icons on the desktop (and no menus or other info at the top of the screen).

The problem started when I had a few browser windows open in Safari and one in Firefox, to watch video. I had been unable to play videos

in Safari, which I now realize I should have paid more attention to. The video froze,

and the sound almost seemed to disintegrate into tinny components.

I believe these are the tech specs for my computer (I got them from an Amazon page I found from searching for the part number).

2.2 GHz Quad-Core Intel Core i7 Processor

4 GB 1333MHz DDR3 SDRAM

500GB 5400rpm Hard Drive; 8x DVD/CD SuperDrive

15.4-Inch LED-backlit Display, 1440-by-900 Resolution; AMD Radeon HD 6750M with 512MB GDDR5

Mac OS X v10.7 Lion