First eliminate all of unused startup programs by going to start->run->msconfig->startup tab and disable any useless programs. Don't disable your Anti virus now go to the services tab, select hide all Microsoft services and again disable all of the useless programs there.
If that still didn't help you, then get Microsoft Bootvis. This program was made by Microsoft to reduce boot up times from 30seconds to 20seconds. I found this program especially usefully when you have a lot of programs at startup.
Get it from here.
If that still didn't help you, then defragment your hard drive by going to start->all programs->accessories->system tools->disk defragmenter .
Now defragment your hard drives or partitions, this will make your files and folders easier to read resulting in higher performance.
Now after all of that and it still didn't help then you need to check your system for viruses, spyware, adware, malware, etc.......
I hope this helps.
Have a nice day :)
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