Why your computer loads slow can be attributed to a few reasons within the Windows operating system and because of some types of programs that clog up your memory and CPU usage that slows the computer down when loading to Windows and then when you log into Windows as well . The three main reasons why your computer loads slow are as follows:
One reason your computer loads slow is that you may have some software that runs on startup that consumers and amazing amount of power leaving nothing for other programs and the user. Most often this culprit is an anti-virus checker that scans your startup and files every time you start your computer causing it to load slow. To combat this you can often disable the startup check or even disable the entire program and only use it manually if you feel secure enough as many virus checkers are overkill.
Fragmented Hard Drive
When data is moved around your computer and programs and files are deleted, installed, changed and manipulated it causes an issue called fragmentation which is when data is all over the hard disk and simply put, Windows has to spend a lot of time following references and links to all parts of the hard drive to collect all the data needed to run itself and other programs. This can cause your computer to load slow and continue to be slow when in operation as well. To fix this you can go to the System Tools in Windows which has a program called 'disk defragmenter' which finds all these fragments and sorts them in order creating a faster running hard drive. Be warned though a badly fragmented drive can take hours to defrag!
One problem which is harder to fix and is a major reason your computer loads slow is that of registry errors. The registry is where Windows stores all the software and hardware instructions to run your computer, unfortunately over time errors appear often from programs not uninstalling correctly. This is harder to pick and harder to correct that the other reasons but when your computer loads slow even if the other things are fixed this will always be an invisible issue causing performance problems.