That's because software comes from the realm of biology, not physics, improvements are path-dependent, historically determined and can quite easily get stuck at local maxima rather than finding globally optimum states.
None of my multi-GHz, multi-core, multi-gigabyte machines boot up anything like as fast as my old AppleII or Osbourne portable. None of my text editors launches as quickly as WordStar used too. Moore's Law has _never_ quite kept up with the speed that software slows down.
PS: It actually look less time to install windows 7 ~15 minutes than it took to boot one of my old windows 95 machines with far to little ram and a antivirus software.
This might be true for machines that held the OS in ROM, but most current computers load much faster than an IBM PC loading the OS from a floppy disk.
These comparisons don't mean much, because the software can do thousands of new things that it could't, like connect to the internet, wirelessly, at high speeds, play multimedia at millions of colors and huge sampling rates, etc.
It's not like the exact same programs got somehow slower, just the same category of programs. A modern text editor is so much more advanced that WordStart that it's not even funny.
And you might not remember that loading a simple game in a 8-bit home computer from tape (a cassette tape, such as used by Amstrad CPC, Commodore 64, ZX Spectrum, etc) took ages (5-20 minutes), and involved 1/1000000 of the data of a modern game.
None of my text editors launches as quickly as WordStar used too.
Try an SSD drive and you'd be surprised.