Computer Fault

Last night I went to shut my computer down as it shut down the computer fan dtart to become more loud!  It seemed to be ok then I went through and it seemed to be trying to switch itself back on but it was not able to it kept going off and on.  The harddrive light was coming on and off.  There was no beeps from the computer and the screen looked like it was trying to come on?
Also the computer had been running really slow for few weeks.

please can anyone help with what this could be?

Try Cleaning your computer out, as the fans or components may have been unable to operate properly due to overheating or excessive amounts of dust restricting movement.

If the noise was actually coming from a case or CPU fan I recommend replacing it ASAP as that is generally a surefire sign that it is dying.

nice one will try this when i get back from work i thought that but just wanted to get other people thoughts I thought maybe the ram was going along with the fan?  I will try this and update

thanks 

nice one will try this when i get back from work i thought that but just wanted to get other people thoughts I thought maybe the ram was going along with the fan?  I will try this and update

thanks 

Yes, many issues are related to RAM. Download Memtest86+, burn to a CD and boot off it, if possible.

Although RAM would not cause your computer to run slow. With hardware related errors you will either receive frequent errors from programs etc, or your computer will not run - full stop.

ok well i will try cleaning it out it has not been done for 3 months so would be needing one its about 3 years old.  I did try start it up when i switched it off but it just kept doing the on and off it was not even booting up was like flicking on and off within seconds!