Looking for help on Program Responses

Hi guys, last night I started experience the "Application not responding" issues. I only use Mac for 2 weeks or so and I like it so far, this is never happened before even before upgrading my RAM from 250 to 1GB.
Basically what happens, for example I'm using the Finder and at the moment I'm trying to move file to lets say Recycle its stop responding. I'm trying to open other application and it doesn't wanna open, icon just jumps in the dock but not opening. Now I use "System monitor" to stop the processes etc. but this not always working as sometimes I can't even bring System monitor up. I ended up manually shutting down my Mac couple of times and I assume this is not the best way but I had to do it.
Any suggestions would be greatly appreciated. What else I can do to restart or stop not responding program in Mac, how to do force restart?
p.s. I don't use Mac keyboard so some of the shortcuts might not work for me.

TAIA

 

I installed RAM this past weekend. I spend few days on this Mac after without any problems. So I don't think this is RAM directly unless its just went bad in 4 days

 

Thank You for answers, I think I found the problem. I had my old laptop hard drive attached to my Mac thru USB port and when I tried to access it or write to it this happened.
I just disconnected the HD and the problem went away. I'll see how it’s all going to work now and let you know.

TAA