Hmm. It's a bit difficult to diagnose the snag. Most likely a failing PSU. The only way to prove it is to substitute the existing one with a known good one. They can be had for 20 quid or so, and changing them is pretty straightforward (the various plugs will only fit into the correct sockets). More advice if you feel up to trying this (and in the meantime, we'll see if anyone suggests another route).

PSU = Power Supply Unit. If that is the problem, you won't lose any data by changing it. The symptoms indicate it may be a PSU problem, but not 100% sure. Have you anyone handy who is happy with 'fiddling' with the innards of a PC?

Plugging unplugging USBs when they are transferring data can damage the device plugged in, but not the PC. Ethernet should have no effect. If you're happy to try, disconnect the mains, take off the side panel and unplug any dvs , floppy drives etc, leaving just the main connection to the motherboard and the one to the hard drive. Try booting again and see what happens.

Try disconnecting all devices that are connected to the PC with the exception of Mouse and Keyboard, and see if it starts.

If your PSU is on the way out, these devices may be taking too much power and not allowing the machine to start properly.

If this works, try connecting then in one at a time (reboot after every one) until you find the problem.

A PSU is quite easy to fit, however there are a lot of cables inside your PC and unless you know what you are doing, you could cause problems by (accidently) dislodging memory or the graphic card.

I would strongly advise getting a friend (or look in local paper) to fit it unless you are happy working inside a computer.

Terry