I precabled the house before they arrived (since for free they only would provide one jack), but it's too late for that, I guess.
It's probably overkill, but do backup basic files before they start:
In Win 98: start > run > msconfig > general > create backup > ok
Watch as they install the hardware and software, in case you have to do it yourself later for some reason. Get full instructions and manuals, CD's etc. as appropriate.