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Does it Matter What Operating System I use? (version of windows usually)

The OS or Operating System is merely an interface between the user and the equipment, thus for most users the OS should not matter all that much. 

Some operating systems however do have advantages and limitations, for example Windows 2000 and XP offer much greater network security than any previous versions.  They also offer much more integrated multimedia options, which previously would have to be installed as separate programs.

A word of warning on upgrading- If you upgrade from ME/98/98SE you will have to reformat your hard drive using NTFS to take advantage of network security, reformatting your hard drive will cause you to lose all programs and information.