Nigerian government links with MS
It's unclear whether the Mandriva deal was with the Nigerian federal government, a state government or a local government. If it's the Federal Government then they already have a deal with MS, so they wouldn't pay any extra to install XP on those systems.
Microsoft also encourages the use of its products by providing 'free' training courses for Federal Government employees. It's likely that some money is diverted to senior officials as well, that's just the way business is done there (not too different from here, just more blatant).
There are two good reasons for using Windows in education in Nigeria:
1. Somebody is going to have to maintain those systems. They're much more likely to be familiar with Windows than some Linux distro. Local knowledge of Linux is very limited.
2. Users know what applications they want to use. In particular Microsoft Word and Corel Draw. People want the 'genuine' product, not what they regard as a 'fake' substitute. Windows apps are also what children are much more likely to encounter outside school.
I've just completed two and a half years working in IT in Nigeria. My enthusiasm for spreading Linux was rapidly worn down by the realities there.