I found that I have deleted a very important file. It had been deleted BEFORE the software was installed on my computer. Is it possible to restore the file?
Yes, if the file has not already been written over (by some other files).
If you have found that some important file has been deleted, download and install Active@ File Recovery and search for this file.
It is critical to avoid excessive disk activity on this particular hard drive. Do not intentionally delete any othe files, avoid computer reboots, and do not to run large number of programs concurrently (which would cause the OS to increase Paging/Swapping activity), etc. Any of these might overwrite or partially overwrite the deleted document and also will make finding a particular file more complicated.
The more free hard drive space you have on your computer, the greater the chances for a successful retrieval of deleted file contents.
It is always a good idea to extract and install Active@ File Recovery to some OTHER hard drive - that does not contain important deleted file(s).