Started up my (somewhat ageing) Dimension E520 on Friday evening, after powering it off the night before.  All activity up to that point had been normal, no noticeable problems with the computer.

When I started it up, the power light came on - green, I think, but I will check when next it does this - and three numbers lit up on the front of the unit - 1, 3, and 4. The 1 was blinking and the computer was producing an odd ticking noise in time with it.  Monitor seemed to be receiving no input at all - something I'm familiar with identifying.

I powered it down after a few minutes, and tried to start it again. Same reaction, with the ticking stopping after a few minutes. Eventually, after a few hopeful tries the computer started normally, with nothing unusual barring a system message before Windows loaded stating that the startup had previously failed, mentioning the [WAKE] error code.

Each time I turn on the PC, I go through this process. When the PC does turn on, everything is completely fine - I can find no signs of any problem whatsoever through Device Managers, etc, all the usual methods.

So - I imagine I should worry about this problem, since it can only be a prelude to something worse. But what is causing this, and is it easily repaired?  If it is a hardware component problem - as I imagine it is - how would I go about diagnosing and replacing the part?