Some Sempron's are actually Athlon XP's (Thoroughbread). I myself have a Sempron 2400+, that is actually an AthlonXP with 1667MHz stock speed.
The XP's only support SSE. So trying to run the SSE2 client on such a processor, will result in an error.
Try running a program called CPU-Z (find it via Google), it will tell you what capabilities your processor has.
It should be allright to just install the SSE version on top of the SSE2, restart BOINC, and it should work.