My N900 just developed this problem out of nowhere.

So I try to charge the phone with Nokia's wall charger. What happens is that the LED glows orange for a few times then it completely stops and my phone shuts down.

With the USB-PC cable that came with the Phone, when I plug it in, Windows 7 says that the "USB Device is not recognized" and my phone says it's "charging" but it really isn't because the battery drains slowly as time passes.

What seems to be the problem?

might be your usb port that is coming loose, look to usb fixing thread, or look to threads with the usb coming loose etc

for different charging methods look to my thread (post #2), it's in my signature

Well when I connect it to PC, it somewhat sees the device being plugged in. But it's not recognized. However, with the wall charger, it won't do anything at all.

and that sounds like a broken usb to me, your pc can sense somthing has been connected but if one pin/wire is loose then it cannot connect and gives the not reconized error

have you tryid another usb cable?
of course this will not do much since both the pc-usb cable and your chargers cable doesn't work, wich brings me back to the usb in the N900