I just moved into a new place and got Comcast TV and Internet. The installer put a splitter in my living room for my TV and modem (after some reading now I know I should have asked for two separate lines, oh well). When I went to hook up the 2nd fl TV I realized there was no signal. In the basement, there was an empty splitter dangling from a ground wire attached to the water heater. Is it safe to hook my main "line in" from the outside to this one and split the "outs" to 1st and 2nd floor using this splitter? (A side note, they had Direct TV and all of that junk is still here so I don't know if that splitter previously belonged to that setup.)

It may be OK. That splitter may reduce the signal too low for your internet connection or not have the proper bandwidth. The only splitter installation for internet and tv that I have seen is a two way splitter on the incoming line. One output is for the internet, the other for tv goes to a broadband amplifier and then to a 10-way splitter which goes to almost every room in the house.