Windows Vista

Interestingly, Windows Vista does have a feature called "ReadyBoost" that can "use storage space on some removable media devices, such as USB flash drives, to speed up your computer," according to Microsoft documentation. This documentation can also be found in "Windows Help and Support" as part of any copy of Vista.
"When you insert a compatible device, the AutoPlay dialog box will offer you the option to speed up your system using Windows ReadyBoost," the Microsoft documentation says.
In related news, Intel announced a new Z-P230 PATA (Parallel ATA) SSD drive that comes in 4 gigabyte (GB) and 8GB capacities, with a 16GB version following in September. Pricing is $25 for the 4GB version for 1,000 unit quantities and $45 for 1,000 unit quantities for the 8GB version.