These r the ideas of one of my friend, who happen to mail me with some few project ideas .. exclusively for Computer science students.

1. Sniffer ( C and UNIX) ( Networks)
2. Stegnography - messages in images (C or java and UNIX) ( Multimedia)
Messages can be hidden in images.. You can try to implement any existing algorithm and create a system or you can try to analyse images
for discovering hidden images
3. Buffer overflow ( C and UNIX) (OS) -- First read about buffer overflow.. The basic idea is to detect and avoid buffer overflows.. You
can either do a simple analysis of source code in C to detect buffer over flows and then patch this code.. But this method only detects some as most
of the time the amount of memory allocated will be decided at runtime..
So in another method u try to rectify it at runtime.. But this is difficult..
I will provide more details if you are interested..
4. Coverage analysis of programs ( C or java and UNIX) (Software engineering) -- First read what coverage analysis is.. You can implement it for either C or java..
5. Face detection in images using neural networks and support vector mahcines ( C or java and UNIX ) (Multimedia) -- More details if u are
interested.
6. Character recognition using machine learning
( C or java and UNIX) (Machine learning) --
7. Antispam software -- Many methods are applied for doing this.. You can apply neural networks.. Then the project will be done in C and UNIX and
comes under mahcine learning.. If you use any other method then these will change.. More details if you are interested..
8. File system in file (C and UNIX) (OS) -- I told u about this..
9. Automatic news classifier ( C and UNIX) ( Machine learning) -- The aim is to automatically classify news into various categories like sports,
politics, business, entertainment etc.. Several techniques can be
applied..
10. Design and implementation of a peer-to-peer network ( C and UNIX ) or (VB and Windows) ( NW) -- Create a peer-to-peer network like kazaa..
11. Virtual transport network (C and UNIX) (NW)
12. Thread package (C and UNIX) (OS)