Increasing Computer Security through Hardware?

Audiostar asks: "I am interested in adding some security to several of my computers, but am unsure as to which product to go with. I would like to use some sort of external security measure, such as a pen drive token or something similar. I had considered custom building a key card and reader to install on all my machines, but once I started thinking about the cost and time of building a card reader for each of my computers it became rather impractical. Does anyone have any suggestions for external locking devices or software? I would prefer something that I could use on both my Windows and Linux machines, but protecting the Windows machines are the top priority. I don't need anything too fancy, just an added layer of protection from the multitude of various people who come in and out of my place of business everyday. I own a 128mb flash disk watch, so possibly using that as a token would be both easy and geek chic. Any suggestions on what to install?"