Just installing and deleting programs do not usually slow down a PC.
What does? Installing programs that require (but usually do not need) a program to start up at boot time. Like printers for camera programs.
Two little hard drive space. Easy to check out, right click on the C: drive and select properties. Unused space should be above 20gb or so.
What is slow, internet, or programs that start up slowly?
I have over 200 programs installed, but I get rid of the specific programs that start up at boot time. Someone can help you with that.
Too many emails can slow up gmail or hotmail, delete what you can.
Browsers issues can be popup, ads, or too many ad-ons.