How to hook up a computer to a home theater

I want to be able to input music and video files to my high end home theater. The pre-amp does not have a USB port so I can't use Wifi. Does anyone know a good way (or the best way) to get my video/audio files to my home theater pre-amp?
You are confused I think on what wifi is. You certainly can use it if you wish, you just need a device like the Sonos or Squeezebox, or a USB to SPDIF converter to get the digital out of your computer. Read up on here, there are tons of threads, and if you have specific questions then post and you should get good response.
Cheap way would be from your speaker or headphone jach using splitter that will allow you to go from one jack at the PC to 2 RCA jacks into your pre amp, cheap and reasonable results for audio files.
you can get one of several outboard DACs that accept USB input with RCA out, or you can go with a high end sound card with RCA out. Based on your description of a high end HT, i would recommend the former. There are some very good new DACs that accept USB input.
This is what I use...comes with everything you need.

Several ways, depending on your computer outputs:

USB Port>USB Cable>USB DAC>RCA Cable>Pre

SoundCard Digital Out>Digital Coax Cable>DAC>RCA Cable>Pre

SoundCard RCA Out>RCA Cable>Pre

Soundcard mini-jack out>mini-jack to RCA Cable>Pre
Audio, yes. Video, no. I'm using a LinkSys Wireless MusicBridge that connects via coax digital into my HK pre-pro. I can play any audio file from my PC (via WinAmp) and stream it out to my hi-fi. It cost me $55 and about 1 hour of setup time...

For the best sound, you should get a usb sound card and send the digital to an external d/a converter.

This one is cheap.

This one is expensive, but probably worth it if you have the bucks.

There are also usb d/a converters that do a great job.

The worst sound will come from taking the analog out of a PCI sound card or your motherboard's onboard audio.
Poster check out Roku they have a box that does both audio and video.