Wireless signal overtaken?

I've posted things relating to my wireless signal dropping, but I noticed something that I don't know if it can be done , but seems the reason why my signal keeps dropping off.

I run a Linksys wrt54g wireless router with D-Link PC wireless cards. Lately, my wireless signal has been dropping off and I have tried everything to reinstate it...updating the software, changing the encryption from WEP to WPA and resetting the router. I have been getting a signal from another router in the area that is called default and my previous post said that someone has a wireless that is not secure. I've installed network stumbler to scan for other signals and it came up with my wireless and the default.

It says that the default vendor is a D-Link router. Mine is a Linksys.
Since my PC cards are D-Links, can the other guys D-Link wireless router overtake my Linksys signal? How can I set up my network to only look at my linksys router signal. I have been working at home this week and I just noticed that at 9am this morning my signal dropped after being up all night. I am assuming that whoever is "default" got on at 9am and is interfering with my signal. I have never had this problem before this week and the default signal was never there til this week also. I don't know if it is a neighbor or a local business in this area that is causing me problems.

I am getting pretty frustrated since I do a lot on emailing and online stuff for work.

 

I am deciding to let WINDOWS manage my wireless connection instead of D-Link's utility. The only thing that I can't configure is the WPA encryption. I had to revert back to my WEP encryption settings because WPA is not an aoption in the drop down list in the WINDOWS encryption configuration. I tried to install the XP patch for WPA, but since I already have SP2, it must already be updated.

Somehow, I have my wireless up, but it may go down for reasons I still don't know why. I read through the router's manual and it stated that if I am using XP, then I don't need to use my PC wireless card's config utility to manage my connection...just use Windows to manage it.

Does anyone know how I can configure (thru XP ) my WPA encryption settings? Like I said, it doesn't have the WPA as an option...only WEP or disable. Lastly, OPEN or SHARED auth. mode?

THanks in advance.