- your user must have full administrator rights, otherwise you can't install or un-install a lot of programs. Users of XP have that by default but that can be changed.
See the last part of response 2 in this for the situation for XP:
- If your computer is/was provided by a business or institution, a network administrator may have assigned limited rights to your user, so that you can't do certain things that will possibly mess up their operating system installation Only a user with full administrator rights can change that so your user has administrator rights.
The same can apply if someone else you know owns the computer if they don't want you to be messing up their operating system installation or installing or un-installing programs without their approval.
- some malware messes up your Windows installation such that you can't do things you would normally be able to do. E.g. you may not be abe to run any or some *.exe files. Scan your system with a full scan with whatever anti-malware soiftware you have installed.
If that doesn't help, or instead, try downloading, installing, and running the free version of Malwarebytes.
See response 1:
Ignore the part about loading your latest System Restore restore point unless you did what the guy in that Topic said he did in the first post - loaded a previous restore point.