It is possible for viruses and worms to infect a computer, sometimes from the same source as the pentagrams and Demon sigils. Many of these can be prevented or cured with a good antivirus program. Notably, Demons have nothing to do with these viruses--though the witlesses would make it seem as if Satan was messing up a computer that just has a worm.
However, I don't see any evidence of Demons tampering with my computer. Angels, now that's another story. Angels can get in and mess with your Internet service and do whatever it takes to keep you from getting the real truth about Satan and His Demons. Of course, they blame Satan for this. Often, the idiots will try and purge their computers of Satan, and if it is angels, the angels will cease and desist (though not fix the damage they did) once you return to "the lord". After a while, angels will mess with your computer again to extort more service to joke-hova or Jesus. This continues, and they become afraid to do more than use the computer for Jesus/joke-hova purposes. Even more stupid is when it is simply a worm and they think Demons infested their computer without thinking if it's a worm or unnecessary processes (or registry errors).
Other real problems that afflict computers include incompatible or defective software (usually hogging your CPU), bad sectors in your hard drive, a bad or outdated CPU, insufficient RAM (all 32 bit systems need 4 GB RAM; 64 bit systems do well with 16 or 32 GB or more), outdated or corrupt drivers, just plain filthy dirty innards (you might be surprised, especially if you smoke near your computer), or outdated or corrupt software. Anyone still using anything older than Windows XP needs to update to Windows 7 or 8 (if you are using Vista, an upgrade to Windows 7 or 8 might be a good idea). Your browser should be up to date--IE8 for Windows XP, 9 or 10 for higher versions; or Firefox or Chrome--the latest version). You need a decent anti-virus program, but not more than 2 (one that is good for viruses and the other for spyware is recommended). More than 2, and the risk for conflict goes way up and the benefits do not. A good registry cleaner is recommended. If you are not using a SSD, regular defragmenting your hard drive is a good idea (with SSD, it is not recommended). Keep your clutter (photos, music, videos) on a separate drive, and get rid of crapware found on most operating systems.
Or, if you are afraid of messing up your computer, get a professional (beware that some are scammers). Have a good checkup, and get the crap off your computer. Update your software and drivers, especially antivirus. Get that tiny hard drive replaced--if it's more than 5 years old, it probably has bad sectors anyways. Get plenty of RAM--it will also reduce constant use of the hard drive. Though a good registry cleaner is expensive, it is worth it if you have more than 100 registry errors. Use it, and you will find that those "demon infestations" all disappear even while you get to keep your pentagram and Demon sigils on your hard drive. Notably, I never noticed any problems with my computer that started directly after downloaded those sigils.