Detailed Information

1. Hard disk file fragmentation. See details on why this important, below.
2. Lack of memory RAM. Memory chips are the cheapest way to boost performance. Upgrade to 1GB or 2GB for Windows XP and you will appreciate the faster performance.
3. Overload of start up programs loaded in memory when you boot up. Use The Ultimate Troubleshooter (TUT) to see what you have loaded and remove unnecessary programs and services.
4. Unnecessary security program duplication. Don't use too many security programs. More is not necessarily better. They can fight each other and compete for computing resources. Stick to the best in each class for firewall and antivirus and antispyware shields and scanning programs.

Hard disk fragmentation occurs when users create, delete and resize data files on their computer. The continual creation and deletion of files causes segments originally created in a continuous block to become scattered, or fragmented, into multiple pieces over time as the space they would normally grow over into is taken up by another file. So file growth occurs by breaking into multiple pieces in different locations. When your hard disk needs to read a file, if it is scattered into multiple pieces in many locations, it takes longer for the hard disk to read the entire file. Saving the file over multiple location and pieces is also slower. When you have this repeat for may files, your system performance will noticeably slow down. Now add an already intensive process like an antivirus scanner - and it will take considerably longer to run it's scan - and you will be inconvenienced in longer wait time to use your computer to its optimum.

Antivirus and AntiSpyware scans should be run frequently. If your hard disk is badly fragmented, these scans will slow your system down. Test results, below, show how much faster antivirus scans are performed after a hard disk is defragmented: Source