First, I would recommend that you take a fresh fecal sample to your vet, who will tell you exactly what kind of worms they are. My cat threw up a bunch of roundworms once, and they were treated by Drontal, which works on most kinds of parasitic worms. If you have a multi-cat household, all of your cats must be treated, since one common route of infection is through sharing litter pans. Cats can get roundworms from the feces of other cats, through their mother's milk, or through eating infected rodents, among other routes of infection. I strongly advise you to go to the vet so you know what kind of worm you're dealing with, and get the appropriate medicine, which should be more effective than an over-the-counter brand. When I had my cats de-wormed, they needed only one pill each, and the medication was cheap. Since my cats were born in a small cattery and have never been outside, the probable route of infection was their mother's milk.

Sometimes new cats just want somewhere to hide out and feel safe. Some cats have been known to hide for weeks until they feel safe in their new home and it is best to give them their space. Try luring your cat out with food or treats. Put the food and water nearby and then leave the room. If you see he's eaten some of the food you'll know he can get out OK and isn't in any danger. Giving him some other hiding places around the house may help him come out and explore. Cardboard boxes with a cat-sized hole in the side are good.