Hi baden-baden
This sounds like a hardware issue to me. A PC in storage may have developed some corrosion problems on terminals.
Even with no technical experience the following might help.
Switch PC off, leave plug in (to provide earthing).
Remove casing, first earth out your body by touching a metal part of your PC.
Remove Memory sticks taking care to click back any retaing clips. Check and clean terminals, replace sticks securely.
Repeat this with all other installed cards - VGA/sound.
If all this fails, get your Pc to your local PC shop as you may need a new power supply (not that expensive) - this can be checked by you but is best done by a technician.
Finally software usually complains if it has a problem - have you noticed any warning boxes?
If all you are useing your PC for is the web then an old PC will manage.
If you plan to install new software this is very demanding and you will need a new PC.
Hope this helps