AVG Anti-Spyware (formerly Ewido Anti-Trojan)
The most powerful scanner for Windows that I know of is called AVG Anti-Spyware (formerly known as Ewido). You can download a free 15-day trial version of the program and scan a computer with it. It will often find spyware, viruses, and trojans that all the other programs miss. If you have to use a public computer that runs Windows for any kind of transaction, I recommend at least scanning it with Ewido just to see how infected the computer might be.
If you find a trojan on your computer you should reinstall the computer. Once the computer has been infected by a trojan, it is possible that intruders have gotten into the computer and once they are inside they can make modifications that you will never be able to find with any scanner.
That is one reason why using Linux from a live CD is much more secure. An operating system running from a CD-R cannot be modified and you know it is clean on every boot.