The first thing to look at is anything new added as far as programs, downloads, etc. there since that will create new startups using more resources. After any lengthy period of time after a good number of programs have been added or removed the system registry itself begins to see now useless entries still trying load things no longer installed.
To cope with the average dilemna steps like adding more memory, using the msconfig utility to reduce items in the startup group and even non MS items in the services group, and running a good registry cleaning tool as well as defragmenting the hard drive when that sees a higher percentage of fragmentation occur are key steps. Occasionally the need for driver updates especially where gaming is common can also help.