Comcast might be your best bet, although a friend's recent experience with another company raises doubts. He couldn't get a wireless signal from one upstairs room to another, and the problem was never solved.

If no one here can help, maybe the Computer Help forum would be worth a try.

This is not comcasts problem it is because your router is not strong enough, the new routers are an N, which has the best distance rating. Yours may be one of the earlier models. But you can get a signal bouncer that plugs into your electric wall socket and helps the signal extend. You can use one on each floor. They are relatively inexpensive.
However if your router is fairly old you may benefit more from one of the new ones, plus you want the latest and best security settings.
If your router has a screw on antenna one easy option is to buy a bigger stronger antenna, just Google your router model number and stronger antenna. Some have internal antennas so that is not an option.
You can Google wireless signal bouncer, booster, router booster for the other options. I would look in up for you but I am on my phone right now.

I just checked walmart.com, they have a linksys and a Cisco valet both N wireless routers for under $30, you can get one of those hooked up at the desk which should be plenty but you can use the old one on one of the other floors to really improve your signal there.
I got my linksys at the walmart store I think it was $35.

Your router is not sensitive enough to reach the distance you need. Look up product reviews made by various trade publications. It appears that a twin antenna version has greater distance.

I am running a Gateway by AT&T. Its lcoated in a second story bedroom and serves the entire house.

There is one caveat on going to high power. First, there is a legal limit on how powerful it may be. Secondly, a powerful unit that reaches outside the house can be detected by other computers allowing someone to ride your installation for free. There is a danger of eavesdropping. However, most routers can run encrypted and that is more secure.

Thank you for the suggestions. Our router is old and to make it worse it's in the worst possible location in our home. I will happily give these recommendations to my son and send him off to Walmart.

Thanks again. Maire

In my friend's case, he was renting the router from his phone company. (Don't ask me why.) They ended up sending him three different ones, but none could do the job. I'm not sure, but I think he finally gave up on wireless.

We bought a booster at Best Buy, they have more then one and can recommend one for you. It plugs into an outlet in our bedroom, the wireless router is in our office. It really helped. I am on my kindle fire every evening and my husband is on his IPad, we read in bed.

A power line booster will get the signal to the rest of your house. You can get a starter set with good ratings for $50.