A few ways.
One way would be to delete important system files, especially the
files located in C:\Windows. It's best that one not touch the files
there, unless you know what you are doing.
The second way would be through viruses or worms. Some viruses or
worms can modify system files, causing them to be corrupted.
The third way is defragmenting the PC with programs running. During
defragmenting, a lot of files would be moved. By not closing
programs, some programs might be corrupted in the process.
The last way, and one of the most common way, is updating of
drivers and system. Some patches create more problems rather than
solving problems, so it would be wise to wait a while to see if
others have problems with the new patches before proceeding to
install the new patches.
