Your drive might have an error or you are infected with some viruses. Boot into safe mode and use check disk utility to scan for file system errors on your HDD.
hi,
tuneup utilities is a good registry cleaner too
and avast AV is a (born to kill) program
i use it and its the best if u ask me
u can download the trials from there official sites like : www.avast.com and www.tune-up.com
or take it with registers from torents sites (i like these more)
good luck