First off, have you tried a wired connection? If it stays connected for 30 minutes + , then you can be sure it's the wireless.
If you lose network even with solid ethernet, then it points to your network config or your router. It could also be some obscure software: I had a similar problem that I traced to bad VPN software. I uninstalled the Checkpoint VPN software and the problem went away. So you can try uninstalling or disabling software that uses the network. This is rather advanced but -- try opening Windows Services manager (Start - Run - services.msc) and Stop any network or other services that are not required.
If you are sure it's the wireless, try the usual things: put the laptop next to your WAP (wireless access point, or your router), reorient the antennas, reinstall wireless card drivers, retype your WEP or WPA key. Are you using 801.11b, g, or n? Try forcing a lower speed like g or even b to see if it helps.
There are also tweaks you can do on your router but those are manufacturer dependent. What is your router make and model?