I tried Linux too. I had my XP system with a double boot option and fooled around with it. It’s good but certainly not better than Vista. Just pull the monitor and other equipment off the XP machine, stick a new Vista box in there ready and already loaded and you’re good to go. You will love it. I heard all the negatives and they were all wrong. It is a great system.
These work arounds and manipulating of old machines is silly. I would have spent more to upgrade my old XP system than I spent for a brand new dual core Vista system which I now have. It made no financial sense. And besides a 3-4 year old machine is functionally obsolete anyway. It’s time to move on anyway.
That depends on what you really need. Those shoebox HP machines have 1.7 GHz P4 processors and fast 7200 RPM IDE disks. That's more than adequate for a very healthy Linux system. The gcc and g++ compilers under Fedora Core 7 are leading edge tools. Combine that will good libraries e.g. FFTW and you can do some pretty serious signal processing. They are also quite serviceable as web servers and database servers. The "old" P3 800 running Debian 4.0 serves as the DHCP server for my LAN and as a "build" platform for multiple development projects with literally millions of lines of C/C++.
If your functionality requires the latest high definition video, audio, optical disks and 10Gig Ethernet technology, then you are probably in the market for something fairly recent. Different strokes.