Well you install and deinstall a number of programs. If you read magazines there are scores of Movie traiors which you need to install to preview. So far so good, but that harms your registry with fragmentation, broken links and other errors.
You need Tune Up utility 2008 to fix these errors. Apart from that there are 100's of applications sitting in your start Menu which needs to be removed if not to be started automatically. This makes the booting process damn long not knowing if its due to start menu exhaution or Worms and Viruses may mislead your efforts.
If you have a PC use it to its utmost utility. Try the above one which have many other interesting Facets too.