Windows sees USB blaster but Quartus doesn't, why?

My problem is specific to my main development PC and only appeared after nearly a year of relatively reliable operation.

1. The target hardware hasn't changed (it's still the same PCB assembly)

2. The problem affects both of my USB blasters

3. There's no problem if I use my laptop

4. There's no problem if I use a colleages PC

5. Restarting the JTAG server doesn't fix it.

6. Restarting Windows does fix it (even without a power cycle)

Using a laptop isn't a long term solution, the screen is too small and the compile times are too long.