"Error 25000. Bluestacks currently doesn't recognize your graphic card. It is possible your graphics drivers may need to be updated. Please update them and try installing again. "
or this one:
"Hmm ... It seems BlueStacks App Player Couldn't be installed on this computer."
Or ...
"This installer has an invalid digital signature."
How to fix it...
In short: you have to download the offline installer and hack/crack it with a program called Orca.
We have to edit a key of the installer (msi) "avoiding check before installing" so it does not check the RAM (or physical memory), your graphics card and the integrity check (if you did not modify the installer).
Step 1:
Download the offline installer (BlueStacks_HD_AppPlayerPro_setup_REL.msi - link )
Step 2:
Modify installer with Orca. (The link has the program already modified). Skip this step :P
Step 3:
Install Android normally and enjoy on your computer. ;-)
Final Note: The program in its first start it's going to upgrade automatically. Let it do it.
FAQ
How do I edit the installer with Orca?
by @shats
It is not necessary. I already did it. The file you are going to download is already modified to skip key checks.