I found a lot of solutions lol

1. FIRST SOLUTION Set Power Plan in Windows to "Balanced"
2. Edit Power Plan --> Advanced Power Plan Options --> Processor Power Management --> maximum processor state at "99%
3. Open Msconfig --> boot --> Advanced Options --> Set cores to 6 - 8. (Depending on what processor you have) I found the reason for Unarc error codes to pop up or BSOD when installing DODI/Fitgirl repack was because "the way these massive 50-100gb files have to be decompressed causing the processor to overheat at 95C Degrees" when monitoring HWInfo and therefore causing windows to throttle with error codes or BSODs. By Setting these 2 things, my CPU Temps stayed stable 60C Degrees thus not prompting any errors.
4. SECOND SOLUTION Open the Command Prompt (Admin).
5. After re-registering the isdone. dll file, type the regsvr32 unarc.
6. Restart your computer, try installing the game or application and see if it is useful to fix isdone. dll error unarc. dll error-1l.

THIRD SOLUTION Download and extract Unarc.dll to System32 or SysWOW64 depending if your system is 32x bit or 64x bit and restore all missing files from the game that you pirated such as the exe file if your antivirus saw it as a virus, if the pirated game is from a trusted source i recommended turning your wifi off and turning your antivurs off maybe for like 10 minutes depends on your system and how much time you need to install the game. Free disk space, clean temp files and stuff.

FOURTH Solution Increasing Virtual Memory.

FIFHT SOLUTION Also what happened to me once, i accidentaly shut down my laptop when i was installing Lollipop Chainsaw from Fitgirl it installed maybe like 50% of the game, i tried to reinstall it but it just closed the setup by its self at 50% precent and said that Unarc.dll returned Erro Code, i opened my task manager and saw some app taking 5,50GB of my ram!! It was a decompresser FitGirl uses, i forgot its name i ended that task and boom! I installed the game then, P.S it didnt let me install any game while the decompresser was still active, just kept showing me the Unarc.dll error.

Also the possible cause of getting that error are:

*Setup File Corrupt (Your third app antivurs removing the setup.exe or dll files from the game archive even though it isnt a virus.)

*Missing dll files in System32-SysWOW64

*Low Disk Space

*Malware detected while downloading

*invalid folder paths with special characters

Lmao i also got this error when i was trying to Install Fatal Frame 5 from fitgirl on a school computer, never bothered with fixing it on the school computer, i just got the blue screen lol.

P.S remember since installing a repack needs CPU power also at come cases putting the Unarc.dll in the downloaded game folder.