Most people will be aware now that laptops and most desktop PC's no longer come with serial ports as standard. Desktops are simple to modify as a serial card can just be plugged in internally if required, laptops however do not have this luxury. There are a raft of usb to serial converters on the market at present, some good and some bad, some cheap, some expensive and yes you can get cheap and good in the same package if your lucky.
We have used RS Components Stock Code 514-0850 for a few years now and it installed OK in Windows 7 using windows' own drivers. Throughput can be a bit slow due to the usb/serial conversion but it does the job.
Many converters are also available on ebay for very little money, one that we have used is type 340 and looks like this:
It installs fine in Windows 7 and throughput is acceptable and at the time of writing were only £1.80 including delivery!!.
Let us know if you have any issues with these devices and we will do our best to help.