Connect mic to a mixer with XLR inputs and then connect output of mixer to soundcard input. If you use a multiple input mixer you can also connnect tape decks, CD players, etc.
I use a Behringer Eurorack MX802A. It's about $100, gives you incredible sound quality, and can easily handle all your workstation audio needs. I use mine with 6 total ins and outs (3 stereo pairs) and I still have room for 2 mics. Each mic can have +48v of phantom power (for condenser mics). Pretty sweet stuff. :)
The only way I see where you could plug an XLR microphone on a computer would be using a simple standalone external mixer. Peavey has some great analog boards that you can get that would suit your needs. I actually suggest you get something like the 1002-8 RQ Unity series. I have worked with this 8-track analog console before on some productions, and I was amazed at the quality I got out of it. You can also plug the board directly through your sound cards stereo inputs. Cheap alternative to great recordings...
All these suggestions are ok, but IMOA not the best solution. The mixing boards that are mentioned (ie Peavey/Behringer) are a good solution, but for even less money you can get an optimal solution. These mixing boards are very below "Pro" quality standards. They are at best a cheap entry level mixing board. These inexpensive mixing boards come with inexpensive mic pre-amps, which is noticeable once you record 15 seperate tracks of audio and the noise from the pre-amps add up in your mixes. The better solution is to buy a good channel strip vocal processor, which usually consists of a mic pre-amp, a compressor, de-esser, phantom power, and output gain. I recommend the DBX 286A for a good price/quality processor. For about $200 bucks you get all the tools for recording vocals and other instruments. The pluses are that you get a better mic pre than a cheap mixing board, you get a compressor, which you don't get with a mixer, and also a de-esser. If you have more money you buy even more expensive processors. Goto http://www.dbxpro.com/ and check out their mic/processors to get more information and then shop around if you like, but the 286A is a very good quality mic processor. Alls you need is one of these and you plug your mic in the input and the output of the processor into your sound card. If you can afford it, I would buy the DBX 586 mic processor, which has a digital output and then also buy a sound card which has a digital input. Doing it this way will allow you to elliminate the analog output stage of the processor and the analog input stage of the sound card, each of which adds additional noise to your recordings.
Using an external mixer is your best choice. However if you want to "rig" it:
Buy a female xlr connector (A3-F)
Get some GOOD quality mic cable, like Canare.
Pins 1 and 3 on the XLR should be tied together, and connected to the proper size phone plug's shield connector.
The problem with doing this is impedance mismatch and higher probability of picking up noise. Keep the cable as short as possible.
Connect Pin 2 to the center conductor (hot) of the phone plug. Here is a very poor graphic description:
XLR side:
Pins
1 Shield------------------------------
| tied together ----- Phone plug sleeve
3 Signal Low--------------------------
2 Signal High-----------------------------------------------Phone plug center