unable to start or reinstall windows

hi

i have a dell inspiron 1545 laptop which this morning was still working fine. 

when i tried to start it this afternoon, however, it refused to start. it gives me a message on screen, saying windows can't start and that this is possibly caused by a recent change in hardware or software. what i need to do to solve this problem is insert the windows installation disc, restart the computer and thus recover my computer. i do this, and it simply refuses to read the disc. nothing happens.

now, as far as i know nothing has been installed between this morning and this afternoon. also, a couple of weeks ago, cd's were playing fine on my pc so i don't think there's anything wrong with the disc drive. a couple of months ago (in april or may) i had another problem and got a complete new hard drive from dell, so i'd find it very strange if that was broken again. i did however, in the last two weeks or so, get the message that my computer wasn't shut down properly with the question if i wanted to start windows normally or something else. i hardly ever got that message before but figured it was because i had just closed my laptop without shutting it down, when it wasn't connected to power.

also i found out that my guarantee has just expired, so i can't email the dell helpdesk and as it's the weekend now, can't call them either

does anybody here know what it might be and how i can fix this? a virus?? something else?

thanks!