Often in cases like this, there are a few things that may be causing your machine to lag, and eventually crash.
You already pointed out the obvious, so only leaves with a couple of possibilities.

1- you may have a virus, and it's a good idea to do a scan frequently.
virus cause your machine to slow down a lot

2- you may need to do both system clean up, and DE-fragment.
Also run those two applications often, as the computer gets clogged with unwanted files, again causing the machine to lag.

3- your registry may be clogged.
if that is the case, the only solution here is to run a complete system restore to it's original state.
there are numerous programs you can download to clean your registry, but often leave you with unwanted side-effects, in which a lot of programs may not run properly, or even if at all.

hope this will do.

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