If I am not mistaken, a C64 power supply outputs both 5volts DC and 9 volts AC. Certainly not something you would want to use with the Atari.
The Atari 600XL requires just 5volts DC and many of the stock power supplies were around 1amp or so. Its trivial to build a replacement psu if you don't have one available. That one that Tak pointed out on eBay should work fine.
I would suggest buying an APE interface from AtariMax. It plugs into the SIO interface on the Atari and then either a USB or RS232 serial cable plugs into it (depending on model) and that goes to your PC. Install the APE software and then your PC can act as a huge virtual floppy drive, modem or even printer to your Atari computer. Tons of software available for Atari's 8 bit home computer. Be warned though, a stock Atari 600XL only has 16kb of ram and many software titles requires more. There are several inexpensive ram expanions available for it though. Also many software titles that you may download are in fact the PAL versions and either will run too fast on an NTSC machine, or not work correctly at all.