Step1: Open the Avast user interface as figure 1 shows.
Step2: Go to Maintenance à Virus Chest… You should see the libLtVirt2_DrcRules.dll file arrested by Avast J as in figure 2 , therefore you should release it again using the restore button as shown in figure 3.
Step 3: In order to avoid the same misunderstanding from accruing again, go to Settingà Exclusions, and then add the main directory of the Xilinx software. In my case, it is C:\Xilinx\*
Then open the Xilinx and test your steps.