Orthonormal bases

Hi

If I've got an orthonormal basis (a,b), in E^2, such that vector
a=+/-(20/29,21/29) and vector b=+/-(21/29,20/29), how do I find the number of bases satisfying these conditions?
When I've found an orthonormal basis as above, it will always have positive and negative components like above, won't it!?

thanks