My presario SR1920NX gradually sloed down and

You more than likely will have to reformat due to a corrupt system...I would try first going into safemode (unless i misunderstood ) and to to start,run type "msconfig" and then go to startup tab....uncheck anything you just installed....caution: if you dont know what it is is...windows/system then dont uncheck it because that will actually cause the error you describe. Or try un-installing what you recently install in safemode. if you cant access safemode then use the recovery mode to recover the system files...usually the computer will ask for the windows XP cd. good luck...hope this helps