Top suspects for a no-beep POST failure (presuming your system normally beeps after successful POST) are:
Power Supply - It seems yours is probably fine, since the fans and HDD are spinning up. However, since it's the cheapest of these to test by swap-out, you may want to start here.
Motherboard - Next least-expensive to check. Sometimes a simple visual inspection will give it away, but you'll probably have to change it out to be sure.
CPU - Generally, the most expensive of these. Again, visual inspection of a clean CPU and the CPU socket might give away the problem. As with most parts, replacement is often the only real way to verify this suspect.
If you're unsure of whether or not your system should be beeping on a successful POST, (most do, but some don't) consult the manual for your motherboard.
If your system should beep on a successful POST, and isn't, my suggestion would be to first find or procure a spare PSU and swap that in. If that tests successfully, then you've found your problem. If the test fails, I would swap out the motherboard and CPU together.
If you prefer to homebrew your systems, now's a good time to just re-build the whole rig. If not, or you're short on budget, it's probably time to go to your favorite local computer retailer and pick up a new OEM system.