I'm not sure what you're doing.
To delete a line from a file I would expect somthing along the lines of
Code:
Open the file for input
Open a temp file for output
Do until no lines left in the file
Read a line from the file
If the line is not the one we want deleted then
write the line to the temp file
End if
Loop
Close the files
Copy the temp file to the file.
Now, the code you have currently, looks like it is trying to debug your logic, not do the task.
The code looks like it would wipe out your input file, since you open a second file using sw "Temp.txt" and write to that, and then copy "Testing.txt" to your input file which wasn't written to.
In any case, there seems to be a number of obvious flaws in the code logic, regardless of what you intended.
You have a condition, If isFound = False, then inside that condition you test for isFound = True. isFound can never be True since it had to be False to get to that code.
Your loop, For i = 0 to sr4.Peek-1, might work, I haven't tried it, but I wouldn't do that.
Peek is a function which should return the byte offset of the next character to be read from the file. You're not reading new lines from the file in your loop, so the Peek function should never return a different number, so will loop some arbitrary number of times (the length of the first line read from the file + 1), but even if you were reading lines in the loop, I would be surprised if it didn't end up not reading the whole file, or reading past the end of the file.