Do not download pirated software or patches from the internet - When you download a crack, you can be almost certain that some bad things will be included.
If you really can't afford the software you are looking for, contact the author and explain your situation. There is a big chance that you get a reduction or that you get the software for free.
Most crackers will include trojans that they have written themselves and are therefore not detected by virus scanners. Many cracks will include spyware.
The trojans allow the cracker to access your computer. If you use your computer to make payments by credit card or to do online banking, the cracker will have access to your passwords and pin codes and will use your credit card or bank account to buy things.
The spyware can break many things in your computer. It can make your browser unusable. There is also a good chance that viruses are included.
Now you may think that serials are safer. You don't need to install anything on your computer to use them. But if you think this, you are wrong. The web pages where you find the serials are a dangerous place. Many of these web pages exploit security holes in your browser, and can install trojans, spyware and viruses on your computer just by visiting the web page.
Just searching on the web for web pages that contain serials can damage your computer, can give the crackers access to your credit card, and can install spyware that tracks everything you do.
Better Safe than Sorry. Alway buy original software.