Few Points to look after
1.Windows Services : Disable all the unneeded services form the services.msc or msconfig, windows used to enable many services by default
2.Disk fragmentation :- If your disk is derangement the it is a good reason to defrag it as fragmented disk is a reason to slow down your desktop/server.
3. Check for spyware and viruses : They can also decrease the performance of your system. make sure you have anti virus installed and it is updated with the latest definition also make sure that your system is patched with the latest patches.
4. Bad Hardware : Check your HDD and RAM . they are also potential cause for slow system speed. HDD may have bad sectors and RAM might be faulty or not compatible to your mother board. in windows blue screen may be happen due this these.
5. Temp Files/history/cookies : Delete them manually or by cc-cleaner software.