If the bar in Windows hard drive space is red, does it slows down the computer to run any software? What can slows down the computer? Too many programs installed?
Having a lot of your hard drive full does not affect performance at all under normal circumstances.
If it is extremely
close to being totally full it can be a problem, but as long as there's enough space for the page file and a few random temp files or whatever, it literally makes no difference at all.
If you have an extremely
large NUMBER of files you can cause delays in file system access simply by making the OS search through big lists of names to find a given file, but this is unrelated to what % of the drive is full, and in any normal use probably wouldn't be a problem. But a drive that is for example 90% full of mostly large files like movies or whatever shouldn't cause any issues at all.
Yes the more programs you install into your operating system, the more chance you have of getting problematic apps or problematic combinations of apps that can cause performance issues.
Also there is some overhead from things like registry bloat when you have lots of apps installed.
I highly reccomend getting out a pen and paper and writing down a list of only the apps you actually use. Then re-install a fresh copy of your OS and ONLY install those apps that you have written down, this will give you the best possible performance for your system.