this is a case of os corruption somehow..!!..may b a virus/trojan or hardware or software is responsible..
all u need is the os boot cd anyway..
(totake data backups u gotta connect the hdd to another pc/laptop, for suchcases a hdd-case is available in the local spare market, it'll makebehave ur hdd to act as an external removable drive)..
this canalso b a symptom of faulty/defective of RAM..pen clean/remove carbon& or oxide formed on the teeth with an eraser n resit it..[it'll bmore prominent, that if itz cause by ram or not,..if during normaloperation, a Blue Screen appears/Memory Dump]
though, sometimes itz also caused by the m/b..
see if ur hdd is making any humming/ticking noise (like 'tuc tuc' or 'kirr kirr')..if so it is likely to be replaced..
if once u can login, do the following:
hv a licensed and UPDATED antivirus installed (i prefer avg/kaspersky, , without updation a full version antivirus is useless)..
download, install n run microsoft kb890830..http://www.microsoft.com/downloads/details.aspx?FamilyId=AD724AE0-E72D-4F54-9AB3-75B8EB148356&displaylang=en
download n run Startup Control Panel..http://download.cnet.com/Startup-Control-Panel/3000-2094_4-10039330.html
..'remove all theunnecessary startup items from here..(if u can't find what toremove,post me/us in ur comment with all the items listed in each nevery tabs herein)..
remove all unnecessary software from add/remove programs..
download, install n run 'Advanced System Care Pro'..http://download.cnet.com/Advanced-SystemCare-Pro/3000-2086_4-10399324.html
disable thefollowing: a) 'Automatically Restart' right click onmy computer->properties->advanced->startup nrecovery->(uncheck on the) Automatically Restart; b)'ErrorReporting' right click onmy computer->properties->advanced->errorreporting->disable error reporting->(uncheck on the) 'but notify mewhen critical error occur'..
Increase the size of Virtual Memory, Initial: 1440; Maximum: 2880
always better to seek professional advice of ur system admin/vendor....