open up a cmd , or command window by typing
cmd.exe or command in run or startsearch
depending on ur windows version , and in the window you will see you can type stuff into it..
depending on where your apache files are located , in this example im going to use C drive and the default path
"c:\program files\apache group\apache2\bin\apache" -k install -n "apache2"
let me know if you need any further help