This isn't what you might think, but the welcome screen has gone completely! after windows boots up, the screen goes black and all you can see is a mouse pointer which you can move. It happened when I scanned disk with spyware doctor and it found a bunch of ads and one trojan (somewhere in windows folder ~system\lsaa.exe i think). So, it said it will be deleted after restart, so i just turned pc off. Then i come back and guess what! no welcome screen! I have made a full back up of that drive a couple days ago, but restoring windows folder does not help.