You should have a folder called ActiveHome Pro under Program Files. If you installed it though, you should have an ActiveHome Pro link under your start menu and All Programs. Even if for some strange reason the program itself was gone, you should at least see that. If you need to download the software again, you will have to get a hold of X10 since the download links used after you purchase the software expire after a week or two. You will also need to have the original activation codes that were e-mailed to you when you bought it. Once you download it again, you should burn the original install files to a CD and possibly even make yourself a text file with the activation codes that you can put on the CD also. That way you will have it if something happens again.
Hope that helps