Do not let storage space stop you from deleting current files such as documents and spreadsheets on your computer. If these files have been around for years, back them up and take them out of your computer. Other than taking up space, you just have to consider the worst scenarios such as hard disk crashes or imminent formatting caused by system software malfunctions that are common today.
Formatting a hard drive is the last resort but is the easiest solution when it comes to program conflicts and crashes. While there are programs that try to help you restore a system, it remains that your PC will never be the same again due to buggy installation. Overwriting may be a good option but you will notice that after you do this, your computer slows down a bit more.
So to address speed and consistency of the operating system of your computer, it would be best to clean it from scratch and start a fresh copy of your preferred Windows operating system. It is a hard decision to make but at times you are left with very few choices to make.
If you have files that have been around for over 2 years which you don’t access, just copy them to a CD or a tape drive. Do not leave them on your PC hard drive for easy access. At times you will never know when you would need to recover them and normally, backups are the best way to safeguard them in case something untoward happens to your PC.