Make sure you're not running other applications besides KVM (or at least nothing too resource intensive any way).
Make sure you're giving it enough memory for newer versions of Windows (at least 2 GiB for Vista and later, though 4 GiB would be better) without taking too much away from the host (I usually avoid giving guests anything more than half of the memory available on the host, and even that's pushing it for me).
If you're able to dedicate an entire disk to the guest, that would be best, and just make sure to let it have direct access to it.
Finally, consider giving a higher priority to the VM, although, in my experience, this doesn't result in any significant gains in performance, but I haven't done any testing to say that conclusively.
Make sure you're not running other applications besides KVM (or at least nothing too resource intensive any way).
Make sure you're giving it enough memory for newer versions of Windows (at least 2 GiB for Vista and later, though 4 GiB would be better) without taking too much away from the host (I usually avoid giving guests anything more than half of the memory available on the host, and even that's pushing it for me).
If you're able to dedicate an entire disk to the guest, that would be best, and just make sure to let it have direct access to it.
Finally, consider giving a higher priority to the VM, although, in my experience, this doesn't result in any significant gains in performance, but I haven't done any testing to say that conclusively.