At first, I'd switch my PC on and it would be in standby mode straight away (monitor light goes orange, no activity from hard disk), but after 10 minutes or so, it would leave standby and boot up as normal. This started happening about a week ago. Now, it just stays in standby as soon as I switch it on. No amount of key pushing or anything helps.

Additional info: This is an old PC I use for playing older games and surfing the net sometimes, it has 2 HDDs, the original 4.3GB and a 1.2GB which I don't use as it doesn't work, so it has no partition or anything, but still gets power.

Are you getting the Initial POST beep? If not, Remove all non-essential components and disconnect non-essential cables (including IDE, floppy, etc.). All you should need to reach the initial boot screen (memory count, etc.) are: the motherboard, CPU, RAM, power supply, and graphics card. If it boots, start adding components one at a time, and boot up each time. When the system won't boot, you've found your faulty (or incorrectly installed or incompatible) component.