It's similar performance to a laptop of that age - pretty much the same parts.
Start out with running ccleaner and a good disc defragmenter to get rid of the junk and optimise the drive.
That should have a significant effect on a machine that has never been optimised.
https://www.piriform.com/ccleaner/download - and defraggler from the same site. Free versions of both.
IE will be faster after that as well, but you are better off using Chrome.
As long as it has enough RAM (4GB or more), it will run Windows 7 perfectly; I would not use 8 or 10.
If it has more than 4GB (or you plan to upgrade beyond 4GB), use the 64 bit version of Windows.
With 4GB or less, the 32 bit version will be slightly better.
The only other upgrade you can do is to swap the hard drive for an SSD. That will make it boot faster and feel a bit more responsive.
It will never be a gaming machine in any current terms but it will make a good general web browsing, word processing & media player setup etc. and should manage old games such as System Shock 2.
(Install the LAV filter pack to allow video playback with minimal CPU load - details here:
http://www.thehtpc.net/htpc-tips-and-tweaks/media-center-lav-filters-and-external-subtitles/
Disable _all_ microsoft stuff in the filter tweaker and set everything possible to LAV).
If you have the remote, that will probably work with the "Windows media center" stuff in Win 7.