Can anybody explain to me what io.sys and msdos.sys are and what do they do.

io.sys , msdos.sys and command.com are the 3 files essential to booting up your computer.Otherwise often called the "system files" If I'm not mistaken io and msdos files are always hidden files even though visible in Windows when "show all files" is selected.

io.sys http://support.microsoft.com/support/serviceware/windows/win95/33bm4cos.asp msdos.sys http://support.microsoft.com/support/kb/articles/q118/5/79.asp?LNG=ENG&SA=ALLKB

I'll add to Glenn's good answer. The io.sys file is software that enables the computer to perform input/output functions. That's what the letters "io" stand for, input/output. It reads/writes to the storage drives, monitor and keyboard. When you turn on a IBM compatable computer, the BIOS looks for and loads io.sys so all booting software have io.sys files. Starting in Windows 95, msdos.sys is reduced to simply giving configuration information for Windows like what directory Windows is in. command.com has most of the important DOS commands stored in it like dir (directory) and cd (change directory). These are called internal DOS commands because they are part of command.com External DOS commands are separate DOS programs that run alone like format.com and sys.com