Mostly your boot.ini file has some Invalid Entries.
Can you post your c:\boot.ini file here?
Type
notepad c:\boot.ini in Start->Run
BTW, The Default content should be (Considering you have installed XP in C[noparse]
[/noparse]
Code:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /usepmtimer
The File will be Hidden and Read only, so remove the Read Only Attributes.
Edit: Official Microsoft Knowledgebase Article. Follow that if the above method doesn't work (which will in most cases)