Could be a hard drive issue, could be a virus or just an old Windows setup that has been stuffed full of programs that run at startup.
Did you do a virus scan? Malwarebytes and a boot disk that will scan the system, Avira and AVG have a free one you can make.
Check either the computer or drive vendor for a disk scan utility to run a disk check on the hard drive. You should also backup your files and internet bookmarks now if it does end up being a drive issue before the thing fully dies.
If the drive is clean of viruses, the disk check comes back with no errors and you check for any junk utilites like driver updators or "system speedup" programs, backup your files, note what programs you have with any setup codes and do a clean...