I take it that you are using a wireless connection which can have interference. Try these 3 steps to make sure you get back up to speed again.
1)Try locating possible devices interfering with your network. There are lots of different types of devices that could be interfering with your network. To list a few, a neighbor's wireless network, microwaves and cordless phones. To determine if any other wireless networks are in your area, download Netstumbler and run it. It will report back any other wireless networks in your range. If there are other networks, make note of what channel they are operating on. If it's operating on channel 6, and you're also on channel 6, you will likely see interference that could result in slow speeds and/or frequent disconnections from your network, so try changing the wireless channel on your network. If you have 2.4 GHz wireless phones in your home, try finding the option to change the wireless channel they operate on. Many 2.4 GHz phones give you this option to lessen interference, but some may still not have that option. The easiest way to tell your (or your neighbor's) wireless phone is interfering with your network is if when the phone rings, you loose connectivity to the wireless network. If yours is the definite cause of your interference, and you can't change the channel, try buying another phone. You can either get one of the newer phones that operate at more than 5 GHz, or go with some older models that operate at 900 MHz. As odd as it may sound, microwaves operate at frequencies very close to 2.4 GHz and can cause interference. Make sure you don't put your AP near a microwave. Make sure you're actually connecting to your network, not your neighbors. Change your SSID to something unique so you never get confused about which network is yours.
2) Run NetStumbler (see link) as mentioned It will report back any other wireless networks in your range.
3) Run Net Meter: (see link) Net Meter monitors traffic of all network connections and displays real-time graphical and numerical data transfer rates. You can see your real-time download and upload rates. You can know how efficient your Internet connection is. Net Meter displays all network activities. You can immediately see if someone is accessing your computer and transferring on LAN or Internet. You also know whether some dangerous or unexpected network activity is occurring while you are online.
What is SSID?
Short for service set identifier, a 32-character unique identifier attached to the header of packets sent over a WLAN that acts as a password when a mobile device tries to connect to the BSS. The SSID differentiates one WLAN from another, so all access points and all devices attempting to connect to a specific WLAN must use the same SSID. A device will not be permitted to join the BSS unless it can provide the unique SSID. Because an SSID can be sniffed in plain text from a packet it does not supply any security to the network.
An SSID is also referred to as a network name because essentially it is a name that identifies a wireless network.
This should tell you if you are connecting to the wireless device or some one elses or if some one is using your wireless connection.
As I havent personally used the software I have mentioned you might want to find alternative's. Again I have provided links for Free Wireless Security Tools. I got the info about looking for interference from Broadband Reports.Com under the question Q: Why is my wireless connection so slow? (see link 4) it goes into alot of detail.
I hope that you could follow that and it helps if not. I hope you get your answers and back up to speed.