Are these front or rear ports? If they are rear ports you might have a driver problem or they may be broken. Check drivers CD, do a Google search to see if this is a recurring problem with your motherboard model. If broken contact the vender you bought ffrom and/or the manufacturer.
If they are front ports then you need to check if you plugged the adapter cable into the motherboard. Front USB headers are usually located near the bottom of the motherboard, sometimes along the side the faces the front of the case with the optical drives.
You also need to be sure they are plugged in properly and into the correct header. Some headers look similar to USB headers, especially Firewire headers. Both have two rows of pins, one row with five pins and one row with four pins.
If they are front USB ports and plugged in properly to the motherboard but still don't work that once again means you might have a driver problem or the ports are broken.
Also, the chipset (I think the south bridge chip controls USB) might be defective.
Do you have a working USB mouse connected to your computer, or a working USB keyboard? If so then the ports they are connected to are working fine so they aren't broken and the motherboard/chipset is fine leaving it a problem with either the flash drives or their drivers.
Assuming you have a USB keyboard and/or mouse, if their ports work then remove the keyboard and plug in a flash drive to the same port. If it works then the ports you already tried it with are broken. If it doesn't then there is a problem with the flash drive and/or it's driver(s) or your Windows installation, for whatever reason, doesn't like flash drives.
Problems like this can have so many possible causes... The few I mentioned are just some of them, the most easily fixed.
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