Disclaimer: I'm Jordan Smith, Product Marketing Manager at Xandros. I launched Presto at Demo 09 (demo.com) last week. (And what a great experience that was!)
Presto is a simple Windows utility that downloads as an .exe and installs/uninstalls like any other Windows app in XP or Vista. It gives you the option on boot-up of choosing either Windows or Presto. Booting into Presto gives you quick (i.e. sub 10 seconds) access to Firefox, Pidgin and Skype, plus many other apps you can add through presto.cnr.com. In our experience, the aforementioned apps cover the vast majority of quick, online use cases e.g. updating Facebook, checking Gmail, etc. Shutdown is instant.
Presto is not meant to replace Windows. It's not even about Linux. It's about enabling people to quickly, easily and cheaply turn a dusty old computer into a fast, reliable, easy and secure browsing appliance. There's a strong market for this. I've even made a point about stripping out all the visible OS-like stuff because our users don't seem to want or need it. On the contrary: they appreciate the simplicity of Presto.
I'm very interested in getting your feedback on our beta, mainly to identify where we may have gaps in our hardware support. You can sign up for the beta at prestomypc.com. We'll have the beta up on downloads.com and tucows.com on Monday. It's under 500MB (including Open Office, a large chunk of that) and we're working on a way to make the DL less painful.
As always, I'm open to your constructive feedback at jordan.smith(at)xandros.com.