To clarify the above points:
A lot of Microsoft's software includes downgrade rights - meaning that (provided the licence specifies,) if you have purchased Windows 7, you are permitted to install XP instead[1]. You may need to phone Microsoft to obtain a valid licence key[2].
Despite my colleague BC's reservations, there are several legitimate reasons for downgrading to XP. Of these, the biggest is operating-system lock-in on proprietary or bespoke software. For example in my company, we use a lot of bespoke software that was developed on and for legacy operating systems. Upgrading this software is not simple, quick or cheap. While the process of developing new software was underway it was necessary for us to downgrade a lot of the hardware we bought between (say) 2007 and 2011.
Initially the support burden of Windows 7 was greater than Windows XP, while companies got up to speed on the new O/S. For smaller companies with limited I.T. budgets, often the pragmatic decision will be to skip one or more versions of Windows and thus avoid some of the obvious and hidden migration costs.
XP is going out of support in 2014, so the pressure is on to move all vulnerable systems away from XP. Once security updates are no longer available, any network-connected XP machine is essentially a sitting duck. That said, even this risk may be mitigated by the use of security/endpoint products that keep systems locked down, that still support XP.
References:
1.
Understanding Downgrade Rights2.
Downgrade Rights FAQ
