You can get one custom built in a computer shop local to you.
First, decide what DAW software you want to use. Pro Tools is very persnickity about computer hardware. Avid supports only certain hardware configurations. So if you decide to get Pro Tools you'll have to design your PC around it. PT 8 LE and M-Powered, although they support Windows 7 still have issues with it. PT 9 does too. And they are only 32 bit DAWs. They come with BFD Lite. I never got used to it and the Lite version is so useless that its sole purpose is to get you to buy the upgrade to the full version of BFD. BFD2 is probably for pros who know how to drum since that's the way the demo videos of the product are used. They didn't show even if it had premade loops. Only showed contraptions you could use to make your own loops by drumming them yourself. That was a turn off. I went with Addictive Drums. Others here use EZ Drummer, Superior Drums 2, Steven Slate Drums, whatever.
I'm in the process of moving from Pro Tools 8 M-Powered to Presonus Studio One Pro. If you plan to use a lot of virtual instruments Studio One is better than Pro Tools. I'm probably not like most here. I feel Pro Tools is like the abusive husband. You walk on eggs around him. And just when you have the one need satisfied he gets pissed at something else. Pro Tools has a lot of features that you just shouldn't use because they cause problems. I would suggest a DAW with full ASIO support -- lower latency.
Seriously, you won't find less than a Quad Core to be adequate. I'll give you a list of components, since I just went down the road again with a PC.
* Intel i5 or i7 (depending upon how much money you want to spend) -- trust me you'll be glad you did.
* ASUS motherboard with Intel chipset. Make sure the sockets match. Also should have USB 3 support.
* TWO 500 GB or TWO 1 TB SATA 3GB/sec 7200 RPM drives (you may want to add a Solid State drive for your Operating System itself) -- one of the drives is for your DAW software, plugins, and third party audio programs. The second HD is the one you record to. Audio software just works better this way. Hard drive space is cheap these days and programs keep getting bigger. I'd almost want to look at a 10,000 RPM Raptor drive for your Audio Software but these cost more.
* You'll need a video card. It doesn't have to be great: nVidia G-Force 7800 or better.
* Your recording interface will be your sound card.
* One DVD-R drive.
* One external USB 3 7200 RPM drive for file backup -- NOTE Pro Tools 8 does not support USB drives.
Add a LED monitor preferably 27". and you should be good. You can build this system for around $1400 including monitor.
If you want to go with Pro Tools 9, you should probably buy one of the Avid qualified Dells, or HPs. lol. IF you can find them anywhere, because those are the PCs they support. The qualified PCs have Xeon processors that aren't available anymore. Also Avid tech support costs money for calls outside of the 30 day period. You can buy tech support calls for a year in advance, or you can go on their forums and wade through a bunch of stuff. And if you get PT 8 LE expect to be pretty much ignored on DUC. This is why I crossgraded to Studio One Pro. Presonus has been very proactive in their support of the product. They are at version 1.6.4 right now and version 2.0 will be out they're saying in the fall quarter.
Or a Mac. A Mac will run you $2000 to start. The tower will run $3000.