Which welcome screen ? Vista or the Excel splash screen ?
If it's Vista that is hanging, you could try pressing the F key for the boot menu - (mines on F12 on an Acer desktop) but XP used to be F8 so try F8 first - just keep pressing F8 just after you've switched it on until you get a black screen with a load of options. Using the cursors, select "Last known good configuration" or "Safe Mode" and press enter. Give it 5 or 10 minutes to sort iteslf out - if the first one doesn't work, try Safe Mode. After it's got into Safe Mode, shut it down properly and restart normally. This usually clears things up.
If not, post again and see if anyone else can assist. Regards