I just assembled the Ladyada Tiny ISP for AVR Atmel microcontrollers, http://www.ladyada.net/make/usbtinyisp/. Easy to build, had my 10 year old son help with the soldering, only one burn to show for it.
You have to use a USBTiny with it and a COM bridge that simulates a null modem between two virtual com ports and have it emulate a null modem connection: Ladyada's page did a reasonable job of leading through that: http://www.ladyada.net/make/usbtinyisp/stk500compat.html as an STK500 emulation.
Vista x64 requires digitally signed drivers and prior to SP1 you could disable this with the command
bcdedit /set loadoptions DDISABLE_INTEGRITY_CHECKS
This would appear to not work after SP1, I use the Driver Signature Enforcement Overrider (DSEO) available at http://www.ngohq.com/home.php?page=Files&go=cat&dwn_cat_id=34) Essentially you put the system into Test Mode and create a test signature for the driver. I now run with the text “Test Mode” in the corners of all four of my monitors.
Also ran into problems getting WinAVR to run correctly under Vista x64. Basically you need a new msys-1.0.dll and also I recommend not using the standard installation process as the special charcters “()” in the path Program Files (x86) cause conniptions with parts of the WinAVR app and compliation process. Thanks to MadWizard at http://www.madwizard.org/electronics/articles/winavrvista