laurajane
Try this before you spend the money.
1) Right click wireless Icon on system tray and Open Network Connections
2) Right click on Wireless Network Connection and open properties
3) Scroll second window down to Internet protocol[TCP/IP click on it and click Properties button
4) check on both "Obtain an IP address automatically" Circles.
Hopefully that will do the trick. I had the same problem with my PC laptop for 2 weeks. Just fixed it about 2 hours ago doing the above.
God I love my Mac so much more...
Good Luck
Jester