Computer Won’t Power On

About a week ago I went to turn on my desktop and it won’t power on. I don’t have access to another computer to test my power supply, and aside from buying a new one (I’m a college student on an extremely limited bidget) how would I test to see if the power supply is really the problem? What are some of the other possible causes for this? I have an emachines T1860 purchased in mid 2002. I’ve added a few aftermarket peripherals: DVD burner, second HDD, ATI Radeon Graphics Card, second NIC (the integrated one stopped working shortly after I purchased the computer), and I’ve upgraded the RAM. Everything was added well over a year ago.

Thanks