What device are you connecting to the serial port?
Alternative to serial -> USB is a Expresscard with serial port.http://www.newegg.com/Product/Product.aspx?Item=N82E16839150030
Features: Adds one high-speed serial port to your ExpressCard equipped computer Compatible with high-speed modems, monitoring equipment, serial networking and other serial devices Supports data rate up to 920 Kbps Built-in 128-byte FIFO buffers dramatically increase data transmit/receive speed, especially under Windows multitasking environment RoHS compliant
I've used the serial -> usb adaptors with two devices: (1) A Digital dual trace O'scope for for transfering captured waveforms and (2) a Temperature/Dewpoint/RH Unit for downloading 30 day data dumps.
May be very difficault to find one with a serial port ( might be able to get one custom made). Dell Business laptops may have one.
Usb 2.0 - some adaptors support upto 921.6 Kbps and a few are upto 1 Mbps.