I'm with Nihil on this one,

The biggest problem i find with non compter savy people is that they just leave them on all the time. If its not in use pull the plug!

My cable modem comes with a standby button so when i'm not using the connection, i just turn it off. The very first thing that i would do for your friend is to check for a standby button on the connection, and if it doesn't have one then make sure that they know how to pull the connection out when they're not using the net. This can either be at the wall or behind the back, and if neither of those are accesible, buy a second cable, one from the wall, one from the PC, make sure both are long enough to reach to the open and then you can disconnect there. Cheap as chips!

You'll find that if these people don't have ready access to the PC, depending on what they are using it for, they might just move on to some other poor schmuck. Once that is done you then need to start cleaning. Make sure that you are not connected while cleaning, duh, and then post a HJT log so people here can look at it.

I had a 900Mhz with only 256mb of ram that ran like a dog with XP Pro, (nothing else loaded, just xp) i then stuck another 256 in and it runs like a dream now. So with norton etc loaded your 500Mb could easily be taken up, do you hear the hard drive churning a lot, if so you definately need more RAM, basically if you here it churning regularly it means your ram is full, if its silent then the RAM is probably enough. Also go and find a RAM monitor that way you can tell.

Anyway good luck with it.