1) Auto-Starting Services
Too many services starting on bootup will slow your computer down. Not all services are created equally and performance degredation varies from service to service.
2) Auto-Starting Applications
This is the usual culprit for bootup performance problems. All of these applications are executed when windows starts. In Windows XP you can use "msconfig" to significantly reduce startup times.
3) User Profile Data
If user profile data is corrupt, damaged, or missing it may critically slow down your bootup time. Sometimes it can take ten minutes or more to start a computer with this type of problem.
4) Device Drivers
Poorly written device drivers can slow down system loading time considerably. If you think this may be the case try uninstalling suspected hardware from your computer.
5) Winlogon or Explorer shim components
Viruses and some unorthodox anti-virus products will create substitute (or "shim") programs that wrap system components. In these situations it's inevitable that your bootup time will take longer. Remove the offending anti-virus programs or scan for viruses if you think this is the case.
6) Hard Drive (Hardware Level) Performance
When your computer starts up it needs to access a slew of things from the hard drive. Device drivers and core system files need to be loaded off of disk and into memory before you can start using windows. If your hard drive is aging or just slow this can quickly become a performance bottleneck.
7) Hard Drive Disk Fragmentation
Ideally (and this is true in newer versions of Windows), system files and drivers are shrouded from the woes of fragmentation. However, auto-starting applications are usually not. Severe disk fragmentation can cause a noticeable slow down. Defragment often and be rewarded with a quicker start time.
8) CPU Performance
As well as reducing your normal system performance, a very slow CPU can add lots of time onto your bootup time. For most users, however, this is not the source of their startup troubles.
9) BIOS Settings
Fine tuning the BIOS can shave off several seconds of time by removing unnecessary checks and diagnostics.

About The Author

John Ashburn has been involved in computer technical support for over seven years. He has worked in many positions and is skilled at providing clear and easy to digest information to end users about everyday problems with a unique minimalistic writing style.
John occasionally writes for the popular computer repair portal pcfixtool.com.
Read more of his work at: http://www.pcfixtool.com.