Have you tried a complete power down of the broadband box? A few weeks ago I had something similar, a number of sites went real slow on me, and it turned out that some of the translation data in the broadband box had got screwed. Power down and reset forced to to reload, which sorted it out for me.
How much data is there in your temporary internet files folders. If these have got really big, it might be causing memory issues with cache space. If there's a lot in there you might want to look at thinning out some, but exactly what I can't advise, as each machine will be different.
Another one worth checking might be to see if a defragment is needed, if the virtual memory area is badly fragmented, that can cause things to go slow.
Last would be a scan for adware and the like, just in case something nasty snuck in recently. There was a DNS hijack recently that meant that certain machines ended up going to "strange" sites when directed to normally trustworthy sites, and once there, undesirable things happened!