What other info can I offer?
Hostnames and MACs if you have them would be very useful.
So would this person have been able to access my files, or just use my connection?
The person is connected to your internal LAN in exactly the same manner as your own machine is. They certainly CAN access your files, and probably have been inside your router as well. Change your router and Windows passwords immediately. Remove any shared folders and drives from your machine, even the $ hidden admin ones.
Even more worrying for you, as far as the outside world is concerned, anything that the intruder did on the Net is trackable only back to you. If they used your connection to hack or to download kiddie porn, you are accountable for that.
Make no mistake, if you have an intruder then this is very serious.
Tell me more about where you live - do you live in a house on its own, or a flat? Do you have neighbors close by? Likely this is all because one of them has been using your Net connection to surf for free, they used too much bandwidth so you noticed the slowdowns.
This is a wireless router using WEP
WEP was cracked recently. Air-Crack is now able to beat it in a couple of minutes or so on a 1.5 GHz pentium.
You need to go out and get a WPA enabled router, and you can't use wireless any more until you get that.
WEP can be easily cracked !!!
http://www.cdc.informatik.tu-darmstadt.de/aircrack-ptw/
We were able to extend Klein's attack and optimize it for usage against WEP. Using our version, it is possible to recover a 104 bit WEP key with probability 50% using just 40,000 captured packets. For 60,000 available data packets, the success probability is about 80% and for 85,000 data packets about 95%. Using active techniques like deauth and ARP re-injection, 40,000 packets can be captured in less than one minute under good condition. The actual computation takes about 3 seconds and 3 MB main memory on a Pentium-M 1.7 GHz and can additionally be optimized for devices with slower CPUs. The same attack can be used for 40 bit keys too with an even higher success probability.
Go here
http://www.aircrack-ng.org/doku.php
If you want to read all about hacking WEP. WEP is basically worthless now. All you need is a cheap laptop, maybe Linux, a network card, aircrack and about 1-2 minutes of your time.
If you have a WEP only router, replace it with a WPA enabled one, unless you live in a place where no one could be war driving you.