Windows restore/reinstall question?

Question

Everytime when i am attacked by a virus or spyware, my antivirus or antyspyware will ask me if i want to delete the infected file, then the question is...it will delete only the virus/spyware? or the entire file? For example, normally a rootkit or a virus will attack files from my System 32, then if i want to clean/delete it, it will delete the entire virus together with the file which is very important for my windows?

Next question, if the antivirus deletes the file, then that means my windows won't work properly right?
That means i need to reinstall my windows again?

If i reinstall my windows, will i lose any files i had before?
for example, i have a photoshop installed in my pc, it will my disable my photoshop?
I want to recover missing files, but i dunno if there's something missing. Why do i suspect? Because my pc is running slow~
Is there a way to check if i'm missing some files?

One last question, what is a " ddl " file? What happens if i delete them?

Thank You~

Answer 1

some viruses cannot be disinfected from the file and so it is deleted. Windows has copies of most of the important files in C:\WINDOWS\system32\dllcache and it will ask you to recover one from there or automatically fix it if you accidentally delete an important one.

if you reinstall windows, you lose all of the data that you had prior to installing it, with one exception, if you do an "in-place upgrade" which is also called a "repair installation" ( http://support.microsoft.com/kb/315341 )

you will likely need to reinstall photoshop if you reinstall or repair windows

a ddl is a database file, but i think you meant "dll".

below from http://www.filext.com
A .DLL file is a support file and is used by one or more programs. As an example, if several parts of a program need to perform the same action that action may be placed into a .DLL file (library) that the various program parts can all use. This saves space and makes it easier when that particular routine needs to be updated. You should never have a need to "open" a .DLL file on its own; indeed, there really is no way to do so. Note: This file type can become infected and should be carefully scanned if someone sends you a file with this extension. If you want to look at the resources in a DLL library you can use a program like PE Resource Explorer but BE CAREFUL as messing with a DLL file can seriously damage Windows or programs if you do the wrong thing.

Answer 2

It will delete the virus.
If you are getting that many virus' maybe you should install a blocker for this. Virus Protection!!!!
If you crash the system (start over from factory settings) then you will loose everything on the computer.