Windows 7 just like older versions of Windows defragments the system files at startup.
If you have installed Windows on a SSD drive you will no longer need this! Because SSD drives can read files from random locations instantly unlike Harddisks where the heads need to be moved mechanically to different sectors on disks.
For HDD Windows installations you can enable here in registry with Regedit:
Registry Key: HKEY_LOCAL_MACHINE\Software\Microsoft\Dfrg\BootOptimizeFunction
Value Name : Enable
Value Data Type: String Zero (REG_SZ)
Default Value: Y
(For SSD drives set the Default Value to N) or completly delete the Value Name “Enable” because it may slow down your system startup each time trying to defragment the files on your SSD.
For PC performance optimization you can run WinSysClean