Most 2 year old PCs will NOT have 16550A UARTs. If you do not have 16550A UARTs, then you should contact your PC dealer to get your serial ports upgraded. Now that you have the right operating system software and hardware to support a high speed connection, you need a good modem. All modems are not the same. High quality modems like the US Robotics courier make good business sense, they are easily upgradable, work very well, and are in our opinion are the highest quality and most trouble-free modem on the market, they operate at speeds up to 56k.
Line noise is a big one.
If you don't have a good telephone line than all your efforts will be frustrating. Disconnects are the most common sign of line noise, however, line noise can affect performance as well. Line noise will result in data being retransmitted down your telephone line and it doesn't take too many retransmits before performance drops rather dramatically. If you are experiencing disconnects, or suspect line noise problems, you should disconnect all other phones, fax machines, etc. from the phone line as even if they are not in use they can cause or aggravate problems. Line noise is more common in the winter, when water seeps into cables and splices and creates problems.