For getting the Dell case open... if it's an Optiplex 780...there should just be a latch on the top of the case (when standing it vertically). PUll that back and it should release some latches on the sliding side panel. Slide the side panel back, unlocking it from a set of grooves and lift it up.
As for antivirus/antimalware tools...
Avira for antivirus
Malwarebytes Antimalware (free version)
HiJackThis (use only if you know what you're doing - if not, you can damage a system)
ComboFix (Windows 7 only - again, only run if you know what you're doing)
Laptop: MSI GT70 2OC-059us | i7-4700MQ | 16GB | GTX 770m | 500GB SSD / 750GB HDD | 17.3" | Win10 Pro
Desktop: 4690k | 12GB g.Skill RipJaws | GTX 970 | 520hx | Z87X-UD4H | Corsair Vengeance C70 | Corsair H110 | Acer 25" | Acer 22" | Win10
Mobile: Samsung Galaxy Note 5
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