Hey guys. Lately, my iMac has been painstakingly slowwwwwwww (at start up, with basic functions, and online. I guess it's lack of RAM, but I'm not so sure. What do you guys think it can be?

Thanks!

PS: I noticed something last night. After I was talking to my Dad about "the PC's at school being faster than the iMac", the iMac sped up for some reason. Yes guys, I was having impure thoughts about PC's. I thought I needed one, for now. Maybe an XPS 420...