Project Acer Aspire 5742g - First can't boot, then no AntiVirus

Someone brought me a laptop recently which wasn't able to boot anymore. The issue was solved easily, but after rebooting there was more...

It's all about an Acer Aspire 5742g, around five years old. The computer stops at the BIOS boot screen with the option 'Press <F2> to enter Setup'. Windows didn't boot anymore - at every attempt.

But you could still enter the BIOS and change settings without any freezing or anything. So I took a Live-Windows and booted it from the DVD drive - and it worked. I tested some HDD tools and it was all fine, I also had access to the data, no problem. But it still couldn't boot at all.

Then I opened the laptop and removed the hard disk from its cables to take it out for a few minutes. Couldn't see anything unusual and returned the hard disk back into the laptop. I turned the laptop on again and it suddenly worked, Windows 7 was able to boot.

After a few more starts the issue didn't occur again (even weeks later no problem), but I found out that the anti-virus software wasn't running anymore. And for how long? The owner didn't know. The laptop was also filled with lots of software the user wasn't aware about.

A virus scan with Malwarebytes AntiMalware showed more than 300 infections and we agreed to save the data and reinstall the machine. Better save than sorry.