freezing Dell computer

I have a Dell dimension 3000 series computer. Using windows XP. I purchased it on 4-27-05. I am having alot of problems with it freezing plus I received the blue screen stating primary drive 0 not found once last week. Dell tells me I might need more memory. I hear a clunk just before it freezes. This has an extended warranty on it. Could this just be lack of memory or is it the hard drive going out? The disc space is 19.6GB out of a total of 33.8. The physical memory says total is 52224, available 153096 . System cache is 243332. I am not real experience with computers so can someone give me some idea what is going on? Thanks, Linda