Try copying all GP4 files in CD under Windows safe mode. Also try to install under this mode, if doesn't work then install under normal mode.
If you use Win9x or WinNT-based with any FAT drives avaliable, try rebooting through floppy, boot using CD-rom support, then copy all files in GP4 CD into your disk drive.
If doesn't work, ask your dad to take his hard disk to your frind for awhile, copying all GP4 files in CD from your friend's PC.
If this is still doesn't work, probably InstallShiled code components are corrupt. Try install a program shipped with latest version of InstallShield (usually latest game releases).
If this is doesn't work, just relax for moments, think about what you've done wrong in few days before, take a deep breath, hold for about 5 minutes, and make decision
not to leave GP4 community.
In reality, doesn't matter who's
right but most important is who's
left.