My laptop has crashed and when I try to reboot it

thisis a case of os corruption somehow..!!..may b avirus/trojan or hardware or software is responsible..
all u need is the os boot cd anyway..

probably now u got two way out:
goto some cyber n download win xp pro sp2, using any bit-torrent client(like utorrent) from torrent site (eg. mininova.org so on)..make a boot disk n try..
1) copy the file in windows repair mode or from any identical os n put it in place as stated above..
OR2) reinstall os..
(to take data backups u gotta connect the hdd toanother pc/laptop, for such cases a hdd-case is available in the localspare market, it'll make behave ur hdd to act as an external removabledrive)..

this can also b a symptom of faulty/defective ofRAM..pen clean/remove carbon & or oxide formed on the teeth with aneraser n resit it..[it'll b more prominent, that if itz cause by ram ornot,..if during normal operation, a Blue Screen appears/Memory Dump]
though, sometimes itz also caused by the m/b..

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..'removeallthe unnecessary startup items from here..(if u can't find whatto remove,post me/us in ur comment with all the items listed in eachn every 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..
disablethe following: a) 'Automatically Restart' right click on mycomputer->properties->advanced->startup n recovery->(uncheckon the) Automatically Restart; b)'Error Reporting'right click on mycomputer->properties->advanced->error reporting->disableerror reporting->(uncheck on the) 'but notifyme when critical erroroccur'..
Increase the size of Virtual Memory, Initial: 1440; Maximum: 2880

always better to seek professional advice of ur system admin/vendor....

unfortunately the only solution is for you to get that disc or get a copy of the windows xp disc to repair the OS...