In this order usually...

• Hardware - Hardware is the fastest and easiest thing to check that is usually the culprit. Memory is cheap these days, seeing people that still have 128mb and 256mb installed makes me want to cry. With Moore's law being so apparent in the hardware world, CPUs that are still blazing fast can be bought very cheap as well.
• Software clutter - amount of software installed & running upon system startup impacts performance big time. There may also be malicious software slowing you down.
• Updated drivers - these can give your machine big performance boosts.

A good pass of a Defrag program, CCleaner, and a Virus Scan & Malware Scan is beneficial as well.

In this order usually...

• Hardware - Hardware is the fastest and easiest thing to check that is usually the culprit. Memory is cheap these days, seeing people that still have 128 MB and 256 MB installed makes me want to cry. With Moore's law being so apparent in the hardware world, CPUs that are still blazing fast and can be bought very cheap as well.
• Software clutter - the amount of software installed and running upon system startup impacts performance big time. There may also be malicious software slowing you down.
• Updated drivers - these can give your machine big performance boosts.

A good pass of a defragmentation program, CCleaner, and a virus scannner and malware scanner is beneficial as well.