If you wish to start from fresh, then office is going to go...
If you have the office CD's then your set to go, as they will just valedate via the internet, doesnt matter if you have used them 3 times or more. There is a limit, but i have installed office 2007 now about 5 times, and its activated ok.
If it doesnt, then there is an automated phoneline where you can activate via that.
Most acers would have came with a backup disc? All you need to do is insert the disc and reboot.
The second the laptop powers back up again, on my acer you have to hit f12 for boot options, and then select the CD/DVD drive?
The acer reboot will take you through step by step... Really is easy...