What are the methods for getting an old, used windows P.C. ready for secure Bitcoin usage?
If a reinstall is not an option for you then you should do the following:

Requires: CCleaner


Boot in Safe Mode so that any malware start up entries don't get executed.
Use CCleaner to clean up disk space and registry.
Inspect the Start Up list for any suspicious/unwanted entries and
remove them.
Reboot the computer.
Use any anti-virus to remove any present malware and clean infected
files.


This should clean your old pc.

Source: Experience