Windows 7: Dell Studio 1555 suddenly very slow

I have a 2 year old Dell Studio 1555 laptop with Windows 7 Pro 64-bit. It has suddenly developed some problem. It boots very slowly, freezes a bit at times and then comes back to life and works again.
Just now it has booted and when I try to open event viewer I get error "Event log service is unavailable. Verify that the service is running". This does not happen always, sometimes I am able to open the Event Viewer and see some errors (Red exclamations) but not necessarily the same always.
I also noticed today that even when I had lot of apps running memory usage was not going above 49%. Could there be a problem with RAM (2GB x 2 nos). I tried removing and refitting the RAM but there was no difference in performance.
I checked for virus and malware and this does not seem to be problem.
I also used a blower to try and remove dust (if any) from the blowers.
Kindly advice what should be done.