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Why is my Windows computer running slowly?

If your computer won't start correctly, the reason often depends on what happens when you turn on your computer

  • Unneeded programs 
  • Insufficient RAM 
  • Defective or incompatible RAM 
  • Hard disk errors 
  • Fragmented hard drive 
  • Overheating CPU 
    •

Unneeded programs

If your computer is running slowly, you may need to free some space on the hard drive by removing unneeded programs; see In Windows, how do I uninstall programs?

Insufficient RAM

It will be rare to find a computer in regular use that has any version of Windows older than XP, but many users prefer the older Windows releases for personal use. The following operating systems are listed with the corresponding minimum amount of installed RAM with which the computer will still run, according to Microsoft's support website:

Windows Vista Home Premium; Business; Enterprise; Ultimate: 1 GB

Windows Vista Home Basic: 512 MB

Windows XP Pro and Home: 64 MB

Defective or incompatible RAM

In most cases, a computer with either defective or incompatible RAM will simply not function. Other times, it will give clear and unambiguous blue-screen errors. In a few cases, the computer will function, but badly. The fix is to identify the incompatible or defective RAM and replace it. The best thing to do is to identify the proper amount of RAM for your computer and use only that, and, when problems occur, to run burn-in tests (some free ones are available for download) to ensure the RAM is still physically good.

Hard disk errors

Your disk may contain errors (e.g., file allocation errors), which can cause error messages as well as slow performance. To fix these errors, run ScanDisk.

Note: Be sure to run ScanDisk before you run DEFRAG or the Disk Defragmenter. ScanDisk will mark all the bad sectors on the disk; if you run DEFRAG or the Disk Defragmenter before ScanDisk, it might move good information into a bad sector, causing the information to be lost.

To run ScanDisk from XP or Vista:

  1. Double-click My Computer.

  2. Right-click the icon for the drive you want to defragment [usually (C:)], and then select Properties.

  3. Click the Tools tab.

  4. In the "Error-checking" section, click Check Now... .

  5. Check Automatically fix file system errors and Scan for and attempt recovery of bad sectors, and then click Start.

  6. A dialog box will appear, saying:

    "The disk check could not be performed because exclusive access to the drive could not be obtained. Do you want to schedule this disk check to occur the next time you restart the computer?"

    This message indicates that the C: drive is busy because Windows is running, and the scan runs best before Windows loads. Click Yes to schedule the scan, and then restart the computer.

Fragmented hard drive

A fragmented hard drive will make the computer search the hard drive to find pieces of files. Fix this by using a defragmenter; see In Windows XP and Vista, how do I defragment my hard drive?

Overheating CPU

Every Pentium-class and newer computer has fans and heat sinks specifically for cooling the CPU. When the fan wears out or becomes ineffective because it's clogged with dust or other debris, the temperature of the CPU rises. In most cases, the computer simply stops operating, but in some cases, the whole computer will appear to be running slowly or taking long pauses. Often this happens because the CPU is repeatedly locking up for a few seconds at a time. Sometimes it happens when the CPU is running just hot enough to affect its performance but not hot enough to lock up. Either clean the CPU fan so it spins at the proper rate, clean out the heat sink so it dissipates heat properly, or replace the fan/heat sink unit.