Hardware doesn't get slower with time, unless it's failing. i.e. disk drive access times are consistent as a drive ages, provided there is nothing wrong with the drive. And before anyone says "well, it gets slower as it gets more full over time", no, that's not really true either.
New software won't slow down hardware. HOWEVER, much new software will add another component that loads when you start your system, and it's those almost-invisible programs that can cause serious slowness on your system. Most common of these are pre-loads of the program so it will seem to start faster (i.e. acrobat speedlaunch), and those which load updaters and/or "customer experience" components (again, Acrobat which loads its updater, java, MS Office, flash, etc.) Get a whole bunch of these check-for-updates programs running every time you start Windows, and each of them nibbles away at the available resources until things get visible slower & slower.
Also, various antivirus programs may slow your system, and sometimes a new release will cause visible sluggishness.
Last, since new programs tend to presume recent and more-capable hardware, running software built on this premise may cause serious slowness if your old system has lower-performance specs. i.e. many of today's programs presume you have at least 4GB of memory installed, but 2GB was common and many systems still have it, processors were slower etc. So when you use that old system to run what might be a trivial program on a brand-new and more-powerful system, you may see slowness. It is comparable to trying to view Hulu over a dial-up modem -- resource-intensive software which is run on less-resource-providing older hardware -will- be slower than on more-capable hardware (new or not).
But the basic premise of your question is misplaced. There's nothing in newer software that is somehow making that old hardware slower; most digital components simply perform consistently over the years. If your components are healthy, performance will be consistent. If this were not the case, file servers would not be useful for their typical 5-10 year lifespan.