From newest to oldest desktops:  
 
1.  Main Daw
2. Back up Daw
3. Family office and music movie workstation
4. Put in storage for backup
5. Recycle or give away 

Just upgraded my DAW PC from the 8 year old XP one. The wife now uses it for Facebooking, emailing and printing stuff. It still has Sonar 8 on it, so I can use it as a back-up if need be.

Right now I only have 2 PC's .
One is a Win 8 laptop that happens to be my current placeholder DAW …
The other is my P4 running XP . This machine was used as a DAW only that spent its life off line except for updates .
I pulled the Audiophile 192 out of it and it still has a Delta 1010lt in it as a primary sound card .
I put that one in a box with all the peripherals . 
I'm very fond of it , we have spent may enjoyable times making music together .
If I had to go back to using it , I could in a heart beat .
 
Kenny

I have a fast quad core 64 bit machine running Win 7 but I still use my previous computer which is also a Win XP P4 32 bit machine.  The thing to do is to integrate it into your system.  I have a nice audio card in it as well and a midi interface.  So I have audio and midi between the two machines.  Not only that I have them networked as well.  That works great. 
 
Reaper make a set of free plug-ins which you can download and install.  One of them is called ReaStream.  There are 64 bit and 32 bit versions of that plug so it can be installed on both machines.  It allows you to send audio to and from the second machine rather nicely and the latency is low as well.
 
I often use the XP machine just a virtual instrument.  I still have over 70 virtual instruments on that machine. Many are 32 bit only and not available as 64 bit versions so they cannot be used on a 64 bit machine.  I now have full access to them.  Many VST's are stand alone so no host is even needed.  Evan a single core 3 GHz processor can do either one or several virtual instruments at once standing on its ear and they sound great.  I have got some great reverbs and things on it too so it is handy for just an extra reverb etc..
 
Also that machine has a UAD 1 card installed too so I can send audio to and fro and use the UAD stuff as well.
 
I have got Cool Edit Pro installed on it too and nearly invested in Adobe Audition for the 64 bit machine but that would have cost.  All I had to do was network them.  I can open anything on the Win 7 machine from the XP machine.  I can edit files and save them back to the 64 bit machine easily.
 
You dont have to turn it on all the time either just as you need it.  But you do need some sort of mixing system though to bring all the audio back into your mix. 
 
I have a third older Win 98 machine too which is also connected up to my system.  It runs perfectly and has many VST's and plug-ins on it that are nowhere else.  There are 26 virtual instruments on that machine.  I also have a 4th computer too.  An Apple laptop which is fast and powerful.  I have it in the system as well.  The 4 computers all on the same page is amazing and very powerful.  Between them they can handle huge and complex mixes yet none of them are actually working that hard.  I still only do all the sequencing on the main machine though.  I can send midi clock and MTC between them too and lock them up even but I don't do that though.
 
The result capacity of the 4 computers is way more powerful than the sum of the 4 machines.  I run Studio One and have V3 on the main machine but V2 32 bit installed on the XP machine. It is nice if you can organise the same program on both if you can.
 
The other good thing is that not only can the computers work together they can also split up and go their separate ways and do other jobs while you are working on your main machine.  eg remove noise from a long file etc.. or doing backup jobs etc.. It speeds things up.  One can be on line as well or have all your manuals on it too or training tutorials.  Much better to watch something on a separate machine and not take up any real-estate on your main monitor.
 
Don't throw them away. Connect them up and use them. Your system only gets way more powerful as a result.  You don't have to turn them on only when you need them.
 

People think XP was slow,,, but put a SSD drive in your old box and see a huge improvement. XP is still a very useful OS. And I still get security updates for defender and whatnot. It's still sort of supported. 
I just set up an old XP laptop at work we will be using to manage iTunes and a bunch of iPod shuffles. We are initiating the "Alive Inside" program at the care home where I work. 
If you have a relative in a care home you should get into this movement. It's gone viral 
 
http://www.aliveinside.us/
 
 
https://www.youtube.com/watch?v=5FWn4JB2YLU
 
 

Jeff , I enjoyed your post very much  
 
Kenny

Jeff Evans
I have a fast quad core 64 bit machine running Win 7 but I still use my previous computer which is also a Win XP P4 32 bit machine.  The thing to do is to integrate it into your system.  I have a nice audio card in it as well and a midi interface.  So I have audio and midi between the two machines.  Not only that I have them networked as well.  That works great. 
 
 

Thanks for the informative post Jeff. I've often thought about hooking multiple computer together, but I don't know anything about networking computers. Any pointers on doing this?

I run Windows Home Server 2011 on mine. It does scheduled backups of my Studio PC as well as acting as my Web Server. But the backup is the best part.

Yep, give them away to a less fortunate that can use them for basic computing tasks. They just become dust collectors unless you have a specific use for them. Giving them away helps others, which in turn makes you feel good. I'm going out to the shed right now to get rid of those two Win95 machines in the bottom cupboard.