Performance depends if you have nVidia, ATI or intel.
Updated D3D9,10 or 11 should generally be used with ATI and intel.
ATI cards do not use the on onboard RAM with OpenGL.
Depending on how fast and how much system RAM you have left, as to how much slower it is.
Intel disabled several GL functions years ago, so it runs certain features in software mode.
Depending on the speed of your CPU, as to how much this will effect things.
Back on topic, it does seem likely that 1 or both of the previous suggestions are the problem.
Sometimes a task will not finish loading properly when you log into windows, and sits there eating CPU cycles til you kill it.
Try logging off and back in again. This often helps as Windows has less to load.
If you are running low on drive space, then it could be your swap-file killing the performance. If so, a clean-out and defrag are in order.
CPU-z will show you your temperatures
http://www.cpuid.com (PCWizard is it's big brother)
Startup Control Panel, is the most perfect little tool for configuring the startup programs.
http://www.mlin.net/StartupCPL.shtml
Process Explorer, Task Manager replacement can show the CPU usage of individual tasks much better.
www.sysinternals.com
Cacheman, is perfect for tuning Windows memory usage and switching off pointless services. It also memorises "sticky" priorities (useful to tame CPU hungry tasks)
http://www.outertech.com
Tweakers and diagnostics tools
http://my.opera.com/Unreal-Tournament/blog/tweak-windows