There may be several reasons.Follow these steps.
1]Run ScanDisk.
2]Defragment the Drives.
3]Clean Temp files/Waste files using softwares like GLock Temp Cleaner or UltraWipe.
4]Clean Registry using softwares like RegCleaner or RegSupreme or System Mechanic.
5]Close the unused/less used services/processes running in background.
6]Since u have 'less' RAM,increase Virtual Memory/Page File Size.
7]If u are using XP switch to Classic View and disable some of the special effects (Right Click My Computer go to Properties,there go to Advanced tab to make changes.)
8]Scan for Viruses/Trojans using any updated AV.