You cannot mix OSses with BOINC. WU's for Windows need to be processed by the Windows app... the Linux WU's by the Linux app.

You're welcome.

To add something I edited into the wrong post relating to the affinity discussion:

In case you wish to dual boot a 32 bit and 64 bit of the same platform, e.g. Windows 64 and 32 bit, install the lowest bit client i.e. the 32 bit as you wont be able to run 64 bit projects when booted into the 32 bit platform (they'll crash). The rest is piece of cake. Install 32 bit BOINC client both under 32 and 64 bit and choose during the install the same data directory location. Don't think permissions will throw a curve ball, but choose all-users anyhow, ideally for both OSses the same user names/passwords being used.