How can I do a CPU cache flush in x86 Windows?

There are x86 assembly instructions to force the CPU to flush certain cache lines (such as CLFLUSH), but they are pretty obscure. CLFLUSH in particular only flushes a chosen address from all levels of cache (L1, L2, L3).

something as sneaky as doing say a large memcopy?

Yes, this is the simplest approach, and will make sure that the CPU flushes all levels of cache. Just exclude the cache flushing time from your benchmakrs and you should get a good idea how your program performs under cache pressure.

Fortunately, there is more than one way to explicitly flush the caches.

The instruction "wbinvd" writes back modified cache content and marks the caches empty. It executes a bus cycle to make external caches flush their data. Unfortunately, it is a privileged instruction. But if it is possible to run the test program under something like DOS, this is the way to go. This has the advantage of keeping the cache footprint of the "OS" very small.

Additionally, there is the "invd" instruction, which invalidates caches without flushing them back to main memory. This violates the coherency of main memory and cache, so you have to take care of that by yourself. Not really recommended.

For benchmarking purposes, the simplest solution is probably copying a large memory block to a region marked with WC (write combining) instead of WB. The memory mapped region of the graphics card is a good candidate, or you can mark a region as WC by yourself via the MTRR registers.

You can find some resources about benchmarking short routines at Test programs for measuring clock cycles and performance monitoring.