Excellent Connection with 130 mbps but internet still very slow

I had a desktop pc built for gaming at a near by pc store roughly six months ago and placed it in my bedroom. I also have an HP desktop from Best Buy in the living room. The pc in the living room is connected to a cable modem and a brand new Netgear wirless N router. The pc in the living room runs fine and gets excellent internet connection and excellent speed. My sister has a HP lap top that picks up the wireless signal excellent and internet speed is excellent as well but for some reason the internet speed on my pc in the bedroom all of the sudden slowed down tremendously. It has been great up until now. I never changed any settings on the pc and only used it for online gaming. Any ideas on what might have caused this dramatic change to happen? I used to ping in the 30's and 40's now I'm in the 300's and 400's.... I don't get it.....

Intel Dual Core E2180 (2.0G) Cpu 800 MHz FSB
Intel DG31PR Motherboard
3GB DDR2 667 RAM (1X1) (1x2)
160 GB SATA-300 7200 RPM Hard Drive
Palit 8600GT 1GB PCIX Video Card
Level One PCI Wireless N Card
Windows XP Home Edition

The usual cause for high pings and slow speeds is that someone is downloading on the same connection. If anyone has either your upload or download maxxed out, this tends to happen. Torrent files are especially bad because they will easily max out your upload and attempt to do the same to your download. But basically anything that uses a lot of your speed may cause significant slowdown (even high-res video streaming or webcam chat).

There are two other things to consider besides that. One is that some machines that are connected wirelessly may not be seeing the transmissions of the others. If this is the case, lowering the RTS threshold on your router may help (this makes other wireless radios be quiet when packets larger than a certain size need to be sent). This will cause a small slowdown at times, but I'm sure you won't notice it, especially if it fixes your problem.

The last thing I can think of is that you might have the wireless unsecured and someone else has gotten in and is using it for downloading. The best way to secure it is to use WPA2 with AES encryption if you have the option. Also use a very long key of random digits (http://www.yellowpipe.com/yis/tools/.../generator.php). If you find some computers cannot access the internet even though you have put in the correct key, then set the security down to WPA. Do not use WEP except as a last resort, because anyone can crack that.