Do you have carpet? What kind of smell is it? What kind of walls do you have? Just simple painted sheet-rock? Are they dingy?
If you have carpet, I'd be quick to blame that. We rented a house with the worst smell in the world and it was the carpet. We had the carpet cleaned about a million times to no avail. However, this was a really bad case, usually you can fix it.
1. Clean the carpet. You can get an expensive professional service (Shouldn't be too bad for just one room) or rent a carpet cleaner for very cheap. I would recommend using vinegar (I know, i know, it will smell HORRIBLE whiel it's drying, but it REALLY works to neutralize smells after that) or that enzymatic carpet cleaner they sell at pet stores. Those will work WAY better than the shampoo they sell you with the carpet cleaner rental. Also, cover the area in baking soda. Add as much as you can all over the carpet. Leave it on for as long as possible, at least a couple hours, and then vacuum it out. It's a good idea to do this once in a while, but not every time you vacuum or it could hurt your vacuum cleaner.
2. It could be too humid. Try buying a dehumidifier.
3. Wipe the walls down. Test a small area so you don't damage them and either use a solution of warm water and vinegar or a solution of soap and water. Obviously, this will not work with some wall types and textures. Do small areas at a time and place a fan on them if you must.
If you have laminate or wood floors (and it sounds like you do) the source of the smell may be under them. Unfortunately, there's not much you can do in that case. You could get your floors refinished, but that's not cheap and it might not even fix it. If they are laminate, you could rip them up, and then paint this mold and smell stopping primer that you can buy at hardware stores on the underlying surface. This will almost be guaranteed to stop the smell. Then of course, you'd have to put down some new laminate again. Not exactly cheap.
Anyway, good luck. People really under estimate the smell neutralizing powers of baking soda and vinegar. They can work REALLY well, I swear! And vinegar, to most people, smells awful, but I swear it dries scent free. If you can still smell it a long time after cleaning, that means it has not fully dried, which probably suggestions that your room is very humid.
Oh, one last idea. I've never tried this, but some people SWEAR by lighting a lot of candles (any candle, it doesn't matter what kind) and closing all the doors and windows and letting them burn down. Obviously, this is kind of dangerous as you shouldn't be in a room with a lot of burning candles and no ventilation and you should also not leave a room with a bunch of candles unattended. But maybe you could leave the room and just peak in every five minutes? Also, I don't see how this could possibly work, it doesn't seem like there is any science behind it at all, but some people swear by it and say that it's totally saved their houses from smelling too awful to stand, so I don't know. Maybe it's worth a try.
Good luck! I know how horrible it can be. I'm really sensitive to bad smells. :(