My home computer's processor does not report that it can do SSE3. Instead it reports that it can do SSE 1, 2 and 4a.
This causes LHC@Home to not assign SSE3 tasks to this machine. My Intel work machine successfully gets them, so the problem must be in BOINC.

Home computer event log:

7/29/2012 11:32:34 PM | | Starting BOINC client version 7.0.28 for windows_x86_64
7/29/2012 11:32:34 PM | | log flags: file_xfer, sched_ops, task
7/29/2012 11:32:34 PM | | Libraries: libcurl/7.25.0 OpenSSL/1.0.1 zlib/1.2.6
7/29/2012 11:32:34 PM | | Data directory: C:\ProgramData\BOINC
7/29/2012 11:32:34 PM | | Running under account boomm_000
7/29/2012 11:32:34 PM | | Processor: 4 AuthenticAMD AMD Athlon(tm) II X4 635 Processor [Family 16 Model 5 Stepping 2]
7/29/2012 11:32:34 PM | | Processor: 512.00 KB cache
7/29/2012 11:32:34 PM | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 htt pni cx16 syscall nx lm svm sse4a osvw ibs skinit wdt page1gb rdtscp 3dnowext 3dnow
7/29/2012 11:32:34 PM | | OS: Microsoft Windows 8: Professional x64 Edition, (06.02.8400.00)

Prescott New Instructions (PNI) is also know as SSE3

http://en.wikipedia.org/wiki/SSE3

Claggy

And added to that, LHC has PNI and SSE3 apps, for both Intel and AMD processors, so it'll eventually get them. For more information, as ever, see their project forums, more specifically this thread.

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Thanks for the information.