Few days ago I builded the AvrUsb500 STK500 v2 compatible avr programmer from http://www.tuxgraphic.org/electronics/
A very good and relatively fast avr USB rpogrammer. You don't need a programmer to program the ATmega8, it is "self programming". Really system independent (I tried it under Linux and win32). Fully compatible with the AvrStudio (I tried it with the latest version). The communication with the USB port is implanted with the help of an FTDI 232 chip which creates a virtual serial (com) port to your system. Drivers for the FTDI chips are available for any kind of OS (32 and 64 bits, including Windows Vista Linux and MacOs) to the companies site http://www.ftdichip.com/
The only thing that may trouble you is the soldering of the smd FTDI chip. I solder it with a jbc pencil soldering iron using a really small head and thin solder wire. If you have some experience with soldering you wont find it very difficult.
I think that someone could connect the ATmega8 directly to the serial port with the help of an max232 chip and remove the FTDI smd chip. But you will need to find a programmer to program the programmer and you will loose the usb interface. This is only if you want the STK500 v2 compatibility without the difficulty of the smd chip. I will try it the next days.