Since it's still working, create the restore media now. Here's a link from Acer on that.
Also, this has a restore from hard disk recovery system you can use if you don't find what is causing this.
I have Acer Extensa 4630 with Windows Vista Home Basic Service Pack 2 , RAM 3 GB,BIOS version V1.11, 04-Aug-08. I have searched online for a similar problems at boot up but couldn't find any answers. My computer upon booting up displays a black screen with a cursor for a about 2 minutes before getting to the desktop. I tried to boot up in safe mode, still the same problem. I don't have any repair or installation discs to repair or reinstall. Any ideas will be much appreciated.