Macbook Pro takes long to boot up, Freezes easily, Gets Gray Screen saying restart

Question

I've had my Macbook Pro 13 since November 2010, I am still running Snow Leopard, never upgraded or anything. Since last March, my computer takes a long time to turn on, get to the screen where I log in. If I click anything when it gets to the screen right away, I can't click anything, the mouse moves, but nothing responds. I have to wait a good 15-20 minutes before I can click anything. I usually turn my computer on, and leave, come back 25-30 minutes later, and log in. When I have over 3 applications open, ex Firefox, MS Word, Spotify, my computer lags, and sometimes freezes. The freezes last from 30 seconds, to me having to restart the computer. Sometimes the computer freezes randomly when I'm just browsing. Around 70-80% of the time, when I play a video, using flash player like youtube, my computer lags, and after watching a video or 2, the computer freezes. Most of the time, the freeze stays frozen, and around 10 minutes later my mouse can move, but the screen is frozen, and the rainbow loading symbol starts. I just restart the computer. Sometimes, around once every 2 days, or sometimes it happens around 10 times in a couple of hours of me using the computer, my computer gets a gray screen that descends down the screen, it says in multiple languages that I have to immediately restart my computer by pressing the power button down and then pressing it again. Oh, and the bottom of my computer always gets really really hot when I use it.
It is a huge hassle, I can still use the computer, but I would love it if my computer was fixed.

Answer 1

I am still running Snow Leopard, never upgraded or anything.
What about updates?  Check software update.
How large is your hard drive and how much hard drive space do you have left?
Shut down your computer and disconnect all peripherals (keyboard & mouse if pertinent) from your computer.  Now reboot.
If the Mac starts up normally, shut it down again and then plug in one of the peripherals (keyboard or mouse first) and start up your computer again.  If it does so successfully repeat the process, adding one peripheral at a time until your Mac acts up.  At that point, disconnect the last peripheral you added, reboot your Mac and search the peripheral vendor's website for an updated driver. 
If no driver exists or the problem remain after installing the new driver, try a different cable or a different port on your Mac.
If none of the above works, again disconnect all peripherals from your Mac, hold down the "shift" key to start up in "Safe Boot" mode. 
If the Mac starts up correctly, restart without pressing the "shift" key.
If your computer still does not start up properly, shut it down and restart it while holding down the Apple+Option-P-R keys; keep holding "all 4 keys" down until you hear the startup sound "twice."
If none of the above work Disconnect all peripherals from your computer. Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
(If Disk Utility cannot repair, you will need a stronger utility (3rd party) - Diskwarrior or Techtool PRO)
Restart your computer when done.
Repair permissions after you reach the desktop-http://docs.info.apple.com/article.html?artnum=25751 and restart your computer.
Remove any 3rd party ram.
Reinstall Snow Leopard - This will install a "fresh" copy of Snow Leopard without archiving old system files but leaves the rest of your files in place.
Oh, and the bottom of my computer always gets really really hot when I use it.
User Tip:  How to deal with MacBook Pro and MacBook heat?
Per your user manual: Do not operate your MacBook Pro on a pillow or other soft material, as the material can block the airflow vents. Never place anything over the keyboard when operating your MacBook Pro. Never push objects into the ventilation openings. 
From MacBook Pro: Care, Use, and Safety Information
For prolonged use, place your MacBook Pro on a flat, stable surface. Do not place your MacBook Pro on your lap or other body surface for extended periods of time. Prolonged body contact can cause discomfort and potentially a burn.