Dell Vostro 400 problem

Hello,

Having a problem with my work computer which is a Dell Vostro 400, windows Vista. It repeatedly has been crashing overnight and when I come in the morning I have to restart it. That went on for awhile. Months. Last week when I came in it wouldn't start up and was giving giving 6 beeps in a row. I tried many time to get it started, but it wouldn't start and would just give the 6 beeps. It would sound like it was going to start, but then it wouldn't and would give the 6 beeps.

I unplugged it and used another computer and later in the day rehooked it up to another monitor to see if I could get it running. To my surprise it started. Everything seemed fine. The next day I came in, it was giving the 6 beeps on it's own. I had not powered it down, but just logged off. Which is what I always do. So I knew before I even tried to start it there was a problem again as it was giving the 6 beeps. I assumed it wouldn't start, but it did. I'm sure it's just a matter of time before it refuses to start.

Any ideas?