Hi, I have dell laptop with windows xp home. It is not booting up even in safe mode. In safe mode it hangs after displaying loading driver isapnp.sys file. In other modes, it displays blank screen. I tried replacing the sys file from installation CD but no use. I run chkdsk command from recovery console, but no use. I tried repairing the xp using the installation CD. It did copy set of files and restarted after copying. But no use and hangs at the same isapnp.sys files. No error is shown.

isapnp.sys is normally the last driver of "system reserved" group, then xp loads "boot bus extender" group. some virus could have injected a driver belonging to this group: if your antivirus catched it or something weird happened, your xp should hang just at that point.

you have to work offline (with a live cd as ubcd4win or mounting your harddisk on a working pc, not as the bootable one obviously), check if there is something strange in windows\system32\drivers, e.g. a file .sys 0Kb long with an unusual name, then find its name in registry (it could have a bit weird registry entry too)

beware of doing a backup (exporting in a reg file) before deleting those entries. besides that, this type of viruses usually load something else, at least as autorun (e.g. under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run), so you've to check the registry for other suspect entries in known places (and/or doing an offline scan with an update antivirus)