Hi,
I just want to know whether watching movies and videos on PC and storing them on hard disk will affect the performance of PC in any way

No it will not.

Might fill up your hard drive if you find plenty of stuff to record. We should be so lucky.

You should buy the external hard disk. IF you save all movies on your pc's harddisk , It will fill up your internal harddisk.

For me , I bought 250 GB external harddisk and save all the movies in there. So we can use pc's internal harddisk for other purpose like install some software , games , etc..

Thanks
ABB