ok you say you have a start up disk?
put your start up disk in your computer start up your computer then wait till the computer displays start computer with cd rom support then say yes to this.
computer will then go through various checks when it has finished you will get a command line
your cdrom drive is E
at the A: put format C: (A:format C:)
computer will say all information on drive c: will be lost proceed yes/no
it will take a while to format depending on speed of computer and hard drive size
when it has finished formatting it will ask for a name to call your hard drive or you can just press return.
now make sure your windows cd is in your drive
and at the A: prompt put E:
at the E: prompt type setup.exe (E:setup.exe)
windows will guide you through the rest of the setup process.........make sure you have all your drivers disks etc handy.
hope this helps :-)