What's wrong with my wireless connection?

I have used a wireless network on my pc for several years and it's never been great - losing signal occasionally or going slow - but lately it's become much worse. It is usually slower than it used to be and it frequently loses connection (can't resolve the IP address I think is what it said). I don't know anything about networking so I really want to know:
What causes the IP failure?
What can I do to prevent it and increase speed?
Will buying a new wireless adapter help, maybe of a different brand (currently on Belkin as is the router)?
All I know about the network is that my adapter is 54mbps and I think the network is too

  mgmcc 11:26 30 Mar 10

It may be that there are several wireless networks in your neighbourhood and this is causing interference with your connection. Try changing the channel number that your wireless router uses.

To do this, connect your PC to the router by ethernet cable, NOT WIRELESSLY, and type its IP address of into your web browser. This will open the router's configuration pages. Within the wireless settings there will be one for the Channel number. The default for most routers is usually 6 or 11 so, if already using one of these, try the other. Your PC's wireless adapter will pick up the change automatically, so you don't need to change anything there.

Is your wireless network "secured" using WEP or WPA encryption? If not, you may have "unwanted guests" using your wireless connection illegally and this will slow it down.

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