Compared to wired networks, a lot more variables affect the speed that you can achieve on wireless networks.
Generally, on wireless networks the closer your computers are located to each other and the wireless router, the greater your transfer rate. So transferring files between computers in different rooms is usually slower than transferring files between computers in the same room. This is because 802.11g (Wireless-G) signals have some trouble travelling through solid objects like walls, furniture and the like. Also, Wireless-G operates on the 2.4GHz spectrum, so you may get interference from Bluetooth devices, cordless phones, amateur radio and, even, microwaves because they all operate on that spectrum.
So, I'd recommend the following:
- Move your computers closer to the wireless router or move your wireless router to a more central location.
- Turn the signal strength of your wireless router up.
- Ensure that there are only 802.11g devices on your wireless network. Backwards compatability with 802.11b or 802.11a devices will slow the network down significantly.
- Ensure that nothing else on your wireless network is doing a large data transfer. All devices on a wireless network share the available bandwidth, so if someone else is doing a large transfer, the bandwidth available to you is a lot less.
- Try using a different channel (there'll be a setting in your router's configuration page) because you may be getting interference or pollution from other nearby wireless networks. This is especially true if you live in the inner-city or in an apartment block.