Steam is lagging my pc badly

I can use my PC for any gaming whatsoever outside of Steam. But as soon as i log into Steam with my password, about 5 seconds later, the PC lags and the HDD sounds like it struggling to work. The HDD is working in some sort of pattern, the RED light will stay in for 3 seconds then go off for 1 second,then come back on for 3 seconds. This is constant as does not change. trying to do anything with the PC is just about impossible.
I have a i5-2500k, Nvidia GTX-560, 4Gb RAM, 500gb HDD.
This only started happening at the start of January, and have restored my PC back to them incase it was a driver conflict, but no go.
The nvidia driver i was using was 310.97, but have rolled back to 296.10, which did not help either

Stuck for ideas now...

Have you tried reinstalling steam completely? If it's just when steam starts then reinstalling might help but that issue is unusual for just the software. I've seen PCs get shut down when overwhelmed with the game being played but just steam is weird.

i have not as yet because i did not want to risk losing the games i have previously installed. Will that happen if i uninstall and reinstall?

The lag happens as soon as i log until about two after i manage to exit steam.

If you bought the games through the steam store then you will be able to redownload them.

Steam is such a broken program.
I've had constant issues with it in the past.
So much to where I can't even open it anymore.

I've never had a single problem with Steam even though i hear people bash it all the time

You can save folder "steamapps" in some other place while uninstalling steam to not re-download your games. As for your problem, just try what guys said - perform total steam uninstall, clean its traces in registry and manually delete all steam files.