These days with fast processors and large amounts of memory the performance bottleneck on most computers is disk access. Start a program and watch the Disk Access led come on and wait. There are expensive solutions such as SSD drives and RAID O setups but if you just want to speed up your current PC on the cheap give disk defragmentation software a try. I use and recommend UltraDefrag.
20 years ago it seems like everyone defragmented their disk drives as a regular maintenance routine. Today it seems like most people either don’t know what you are talking about or have just gotten out of the habit. Over time creating and deleting files caused them to get spread out all over your hard disk. When files consist of many little fragments scattered around the disk the computer needs more mechanical work to read them. Because of that the disk access becomes slower.
Microsoft includes disk defragmenting software with Windows but it is quite basic and not very good. I use the free UltraDefrag software on all my computers and they are noticeably faster after running the disk optimize routine. UltraDefrag reduces the file fragmentation, thus it makes disk operations faster. Also the program can optimize a whole disk placing all the files to the fastest part of the disk, its beginning. In contrast with the most other disk defragmenters UltraDefrag can defragment locked files by running during the Windows boot. Actually all files including registry hives and page-file can be defragmented there.
You may do a boot time defrag of encrypted disks, but you can still defrag and optimize them under Windows. This is because lsass.exe, the component of the operating system that reads encrypted disks, is still not loaded at a time of the native applications execution. (I encrypt my laptop as I am afraid of it getting stolen and someone getting into my passwords.)
UltraDefrag runs on Windows NT 4.0, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008 and Windows 7, including all 64 bit versions of Windows. Supported file-systems are FAT12/16/32, exFAT and NTFS.