Single short beep is code for POST (power on self test) passed OK. This is normal and a good thing.
Dell OptiPlex GX150 maybe a 1.0Ghz Pentium 3 CPU with 256MB of RAM. And it could possibly be 8 or 9 years old.
What parts do you have inside your GX150?
What version of windows are you using? Still on Windows 98 or Windows NT 4.0?
Has the system been cleaned lately? A fresh install of the operating system?