The short answer is yes. You can use the jailbreak tool GreenPois0n for your Windows PC as it is a simple and straightforward process.
You will only need to upgrade your 3GS and then jailbreak it. Please follow the instructions bellow to learn how to do it.
Note: this is an untethered jailbreak for iPhone 3GS with the new firmware 4.2.1 done with the newest version of greenpois0n (RC 5) windows jailbreak tool.
First of all a small advise: if you have an already unlocked iPhone 3GS you shouldn't use this solution. If you upgrade through iTunes it will upgrade your baseband, and you will be forced to use iPad baseband trick which can be troublesome in some cases.
If you haven't saved the shsh blobs you should not jailbreak with GreenPois0n. Those of you which don't have the shsh blobs wont be able to restore to 4.1 firmware.
GreenPois0n jailbreak steps:
1. Here are the requested downloads: Click here to download GreenPois0n for Windows PC. Download the 4.2.1 firmware for iPhone 3GS here: link Download Tiny Umbrella for Windows here.
2. Upgrade your iTunes if you have older version. Start iTunes and plug in your iPhone into the computer. When your iPhone is recognized update the firmware version by holding shift and clicking on update. When prompted, choose the ipsw you have previously downloaded (note: download the file with Firefox web browser, do not unzip it!)
3. If you see the 1013 error do not worry. Click OK
4. Start Tiny Umbrella and choose the "Exit Recovery" option. Your iPhone will reboot now and you will see the emergency call screen and connect to itunes message. Turn off your iPhone and start GreenPois0n
5. Again put your iPhone into DFU mode by following the instructions
6. When greenpois0n recognizes your iPhone in DFU mode it will start jailbreaking it. Please wait until this is done.
7. When the jailbreak is finished your iPhone will start and you will see the Loader app on your screen.
8. Tapp the Loader app and choose Cydia. You jailbreak was successful. Your 3GS iPhone with 4.2.1 firmware has an untethered jailbreak.
Additional option: If you want to unlock as well as to jailbreak you will now need to install the Ultrasn0w unlock. Tap Cydia and search for Ultrasn0w. When you find please install it and your iPhone will be unlocked. That is all folks, now you have a jailbroken, carrier unlocked iPhone 3GS on the 6.15.00 iPad baseband.