Can I win some money mining bitcoin with an old computer
The short answer: No, you cannot. Sorry.

The longer answer can be found here: https://en.bitcoinwiki.org/wiki/Difficulty_in_Mining#How_soon_might_I_expect_to_generate_a_block.3F

Applying some common sense: If an old computer could mine for 50 USD / EUR / GBP per month, what do you think how many old computers would your neighbor have bought already and was running in his garage?

Bitcoin mining is only possible on specialized hardware these days. Your milage may vary on very, very recent high end GPUs, but this is far from an old unused PCs CPU power.